FreeBSD/Linux Kernel Cross Reference
sys/fs/super.c

     /*
      *  linux/fs/super.c
      *
      *  Copyright (C) 1991, 1992  Linus Torvalds
      *
      *  super.c contains code to handle: - mount structures
      *                                   - super-block tables
      *                                   - filesystem drivers list
      *                                   - mount system call
     *                                   - umount system call
     *                                   - ustat system call
     *
     * GK 2/5/95  -  Changed to support mounting the root fs via NFS
     *
     *  Added kerneld support: Jacques Gelinas and Bjorn Ekwall
     *  Added change_root: Werner Almesberger & Hans Lermen, Feb '96
     *  Added options to /proc/mounts:
     *    Torbjörn Lindh (torbjorn.lindh@gopta.se), April 14, 1996.
     *  Added devfs support: Richard Gooch <rgooch@atnf.csiro.au>, 13-JAN-1998
     *  Heavily rewritten for 'one fs - one tree' dcache architecture. AV, Mar 2000
     */
    
    #include <linux/export.h>
    #include <linux/slab.h>
    #include <linux/acct.h>
    #include <linux/blkdev.h>
    #include <linux/mount.h>
    #include <linux/security.h>
    #include <linux/writeback.h>            /* for the emergency remount stuff */
    #include <linux/idr.h>
    #include <linux/mutex.h>
    #include <linux/backing-dev.h>
    #include <linux/rculist_bl.h>
    #include <linux/cleancache.h>
    #include <linux/fsnotify.h>
    #include <linux/lockdep.h>
    #include "internal.h"
    
    
    LIST_HEAD(super_blocks);
    DEFINE_SPINLOCK(sb_lock);
    
    static char *sb_writers_name[SB_FREEZE_LEVELS] = {
            "sb_writers",
            "sb_pagefaults",
            "sb_internal",
    };
    
    /*
     * One thing we have to be careful of with a per-sb shrinker is that we don't
     * drop the last active reference to the superblock from within the shrinker.
     * If that happens we could trigger unregistering the shrinker from within the
     * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
     * take a passive reference to the superblock to avoid this from occurring.
     */
    static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
    {
            struct super_block *sb;
            int     fs_objects = 0;
            int     total_objects;
    
            sb = container_of(shrink, struct super_block, s_shrink);
    
            /*
             * Deadlock avoidance.  We may hold various FS locks, and we don't want
             * to recurse into the FS that called us in clear_inode() and friends..
             */
            if (sc->nr_to_scan && !(sc->gfp_mask & __GFP_FS))
                    return -1;
    
            if (!grab_super_passive(sb))
                    return -1;
    
            if (sb->s_op && sb->s_op->nr_cached_objects)
                    fs_objects = sb->s_op->nr_cached_objects(sb);
    
            total_objects = sb->s_nr_dentry_unused +
                            sb->s_nr_inodes_unused + fs_objects + 1;
    
            if (sc->nr_to_scan) {
                    int     dentries;
                    int     inodes;
    
                    /* proportion the scan between the caches */
                    dentries = (sc->nr_to_scan * sb->s_nr_dentry_unused) /
                                                            total_objects;
                    inodes = (sc->nr_to_scan * sb->s_nr_inodes_unused) /
                                                            total_objects;
                    if (fs_objects)
                            fs_objects = (sc->nr_to_scan * fs_objects) /
                                                            total_objects;
                    /*
                     * prune the dcache first as the icache is pinned by it, then
                     * prune the icache, followed by the filesystem specific caches
                     */
                    prune_dcache_sb(sb, dentries);
                    prune_icache_sb(sb, inodes);
    
                    if (fs_objects && sb->s_op->free_cached_objects) {
                           sb->s_op->free_cached_objects(sb, fs_objects);
                           fs_objects = sb->s_op->nr_cached_objects(sb);
                   }
                   total_objects = sb->s_nr_dentry_unused +
                                   sb->s_nr_inodes_unused + fs_objects;
           }
   
           total_objects = (total_objects / 100) * sysctl_vfs_cache_pressure;
           drop_super(sb);
           return total_objects;
   }
   
   static int init_sb_writers(struct super_block *s, struct file_system_type *type)
   {
           int err;
           int i;
   
           for (i = 0; i < SB_FREEZE_LEVELS; i++) {
                   err = percpu_counter_init(&s->s_writers.counter[i], 0);
                   if (err < 0)
                           goto err_out;
                   lockdep_init_map(&s->s_writers.lock_map[i], sb_writers_name[i],
                                    &type->s_writers_key[i], 0);
           }
           init_waitqueue_head(&s->s_writers.wait);
           init_waitqueue_head(&s->s_writers.wait_unfrozen);
           return 0;
   err_out:
           while (--i >= 0)
                   percpu_counter_destroy(&s->s_writers.counter[i]);
           return err;
   }
   
   static void destroy_sb_writers(struct super_block *s)
   {
           int i;
   
           for (i = 0; i < SB_FREEZE_LEVELS; i++)
                   percpu_counter_destroy(&s->s_writers.counter[i]);
   }
   
   /**
    *      alloc_super     -       create new superblock
    *      @type:  filesystem type superblock should belong to
    *      @flags: the mount flags
    *
    *      Allocates and initializes a new &struct super_block.  alloc_super()
    *      returns a pointer new superblock or %NULL if allocation had failed.
    */
   static struct super_block *alloc_super(struct file_system_type *type, int flags)
   {
           struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);
           static const struct super_operations default_op;
   
           if (s) {
                   if (security_sb_alloc(s)) {
                           /*
                            * We cannot call security_sb_free() without
                            * security_sb_alloc() succeeding. So bail out manually
                            */
                           kfree(s);
                           s = NULL;
                           goto out;
                   }
   #ifdef CONFIG_SMP
                   s->s_files = alloc_percpu(struct list_head);
                   if (!s->s_files)
                           goto err_out;
                   else {
                           int i;
   
                           for_each_possible_cpu(i)
                                   INIT_LIST_HEAD(per_cpu_ptr(s->s_files, i));
                   }
   #else
                   INIT_LIST_HEAD(&s->s_files);
   #endif
                   if (init_sb_writers(s, type))
                           goto err_out;
                   s->s_flags = flags;
                   s->s_bdi = &default_backing_dev_info;
                   INIT_HLIST_NODE(&s->s_instances);
                   INIT_HLIST_BL_HEAD(&s->s_anon);
                   INIT_LIST_HEAD(&s->s_inodes);
                   INIT_LIST_HEAD(&s->s_dentry_lru);
                   INIT_LIST_HEAD(&s->s_inode_lru);
                   spin_lock_init(&s->s_inode_lru_lock);
                   INIT_LIST_HEAD(&s->s_mounts);
                   init_rwsem(&s->s_umount);
                   lockdep_set_class(&s->s_umount, &type->s_umount_key);
                   /*
                    * sget() can have s_umount recursion.
                    *
                    * When it cannot find a suitable sb, it allocates a new
                    * one (this one), and tries again to find a suitable old
                    * one.
                    *
                    * In case that succeeds, it will acquire the s_umount
                    * lock of the old one. Since these are clearly distrinct
                    * locks, and this object isn't exposed yet, there's no
                    * risk of deadlocks.
                    *
                    * Annotate this by putting this lock in a different
                    * subclass.
                    */
                   down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
                   s->s_count = 1;
                   atomic_set(&s->s_active, 1);
                   mutex_init(&s->s_vfs_rename_mutex);
                   lockdep_set_class(&s->s_vfs_rename_mutex, &type->s_vfs_rename_key);
                   mutex_init(&s->s_dquot.dqio_mutex);
                   mutex_init(&s->s_dquot.dqonoff_mutex);
                   init_rwsem(&s->s_dquot.dqptr_sem);
                   s->s_maxbytes = MAX_NON_LFS;
                   s->s_op = &default_op;
                   s->s_time_gran = 1000000000;
                   s->cleancache_poolid = -1;
   
                   s->s_shrink.seeks = DEFAULT_SEEKS;
                   s->s_shrink.shrink = prune_super;
                   s->s_shrink.batch = 1024;
           }
   out:
           return s;
   err_out:
           security_sb_free(s);
   #ifdef CONFIG_SMP
           if (s->s_files)
                   free_percpu(s->s_files);
   #endif
           destroy_sb_writers(s);
           kfree(s);
           s = NULL;
           goto out;
   }
   
   /**
    *      destroy_super   -       frees a superblock
    *      @s: superblock to free
    *
    *      Frees a superblock.
    */
   static inline void destroy_super(struct super_block *s)
   {
   #ifdef CONFIG_SMP
           free_percpu(s->s_files);
   #endif
           destroy_sb_writers(s);
           security_sb_free(s);
           WARN_ON(!list_empty(&s->s_mounts));
           kfree(s->s_subtype);
           kfree(s->s_options);
           kfree(s);
   }
   
   /* Superblock refcounting  */
   
   /*
    * Drop a superblock's refcount.  The caller must hold sb_lock.
    */
   static void __put_super(struct super_block *sb)
   {
           if (!--sb->s_count) {
                   list_del_init(&sb->s_list);
                   destroy_super(sb);
           }
   }
   
   /**
    *      put_super       -       drop a temporary reference to superblock
    *      @sb: superblock in question
    *
    *      Drops a temporary reference, frees superblock if there's no
    *      references left.
    */
   static void put_super(struct super_block *sb)
   {
           spin_lock(&sb_lock);
           __put_super(sb);
           spin_unlock(&sb_lock);
   }
   
   
   /**
    *      deactivate_locked_super -       drop an active reference to superblock
    *      @s: superblock to deactivate
    *
    *      Drops an active reference to superblock, converting it into a temprory
    *      one if there is no other active references left.  In that case we
    *      tell fs driver to shut it down and drop the temporary reference we
    *      had just acquired.
    *
    *      Caller holds exclusive lock on superblock; that lock is released.
    */
   void deactivate_locked_super(struct super_block *s)
   {
           struct file_system_type *fs = s->s_type;
           if (atomic_dec_and_test(&s->s_active)) {
                   cleancache_invalidate_fs(s);
                   fs->kill_sb(s);
   
                   /* caches are now gone, we can safely kill the shrinker now */
                   unregister_shrinker(&s->s_shrink);
                   put_filesystem(fs);
                   put_super(s);
           } else {
                   up_write(&s->s_umount);
           }
   }
   
   EXPORT_SYMBOL(deactivate_locked_super);
   
   /**
    *      deactivate_super        -       drop an active reference to superblock
    *      @s: superblock to deactivate
    *
    *      Variant of deactivate_locked_super(), except that superblock is *not*
    *      locked by caller.  If we are going to drop the final active reference,
    *      lock will be acquired prior to that.
    */
   void deactivate_super(struct super_block *s)
   {
           if (!atomic_add_unless(&s->s_active, -1, 1)) {
                   down_write(&s->s_umount);
                   deactivate_locked_super(s);
           }
   }
   
   EXPORT_SYMBOL(deactivate_super);
   
   /**
    *      grab_super - acquire an active reference
    *      @s: reference we are trying to make active
    *
    *      Tries to acquire an active reference.  grab_super() is used when we
    *      had just found a superblock in super_blocks or fs_type->fs_supers
    *      and want to turn it into a full-blown active reference.  grab_super()
    *      is called with sb_lock held and drops it.  Returns 1 in case of
    *      success, 0 if we had failed (superblock contents was already dead or
    *      dying when grab_super() had been called).
    */
   static int grab_super(struct super_block *s) __releases(sb_lock)
   {
           if (atomic_inc_not_zero(&s->s_active)) {
                   spin_unlock(&sb_lock);
                   return 1;
           }
           /* it's going away */
           s->s_count++;
           spin_unlock(&sb_lock);
           /* wait for it to die */
           down_write(&s->s_umount);
           up_write(&s->s_umount);
           put_super(s);
           return 0;
   }
   
   /*
    *      grab_super_passive - acquire a passive reference
    *      @sb: reference we are trying to grab
    *
    *      Tries to acquire a passive reference. This is used in places where we
    *      cannot take an active reference but we need to ensure that the
    *      superblock does not go away while we are working on it. It returns
    *      false if a reference was not gained, and returns true with the s_umount
    *      lock held in read mode if a reference is gained. On successful return,
    *      the caller must drop the s_umount lock and the passive reference when
    *      done.
    */
   bool grab_super_passive(struct super_block *sb)
   {
           spin_lock(&sb_lock);
           if (hlist_unhashed(&sb->s_instances)) {
                   spin_unlock(&sb_lock);
                   return false;
           }
   
           sb->s_count++;
           spin_unlock(&sb_lock);
   
           if (down_read_trylock(&sb->s_umount)) {
                   if (sb->s_root && (sb->s_flags & MS_BORN))
                           return true;
                   up_read(&sb->s_umount);
           }
   
           put_super(sb);
           return false;
   }
   
   /**
    *      generic_shutdown_super  -       common helper for ->kill_sb()
    *      @sb: superblock to kill
    *
    *      generic_shutdown_super() does all fs-independent work on superblock
    *      shutdown.  Typical ->kill_sb() should pick all fs-specific objects
    *      that need destruction out of superblock, call generic_shutdown_super()
    *      and release aforementioned objects.  Note: dentries and inodes _are_
    *      taken care of and do not need specific handling.
    *
    *      Upon calling this function, the filesystem may no longer alter or
    *      rearrange the set of dentries belonging to this super_block, nor may it
    *      change the attachments of dentries to inodes.
    */
   void generic_shutdown_super(struct super_block *sb)
   {
           const struct super_operations *sop = sb->s_op;
   
           if (sb->s_root) {
                   shrink_dcache_for_umount(sb);
                   sync_filesystem(sb);
                   sb->s_flags &= ~MS_ACTIVE;
   
                   fsnotify_unmount_inodes(&sb->s_inodes);
   
                   evict_inodes(sb);
   
                   if (sop->put_super)
                           sop->put_super(sb);
   
                   if (!list_empty(&sb->s_inodes)) {
                           printk("VFS: Busy inodes after unmount of %s. "
                              "Self-destruct in 5 seconds.  Have a nice day...\n",
                              sb->s_id);
                   }
           }
           spin_lock(&sb_lock);
           /* should be initialized for __put_super_and_need_restart() */
           hlist_del_init(&sb->s_instances);
           spin_unlock(&sb_lock);
           up_write(&sb->s_umount);
   }
   
   EXPORT_SYMBOL(generic_shutdown_super);
   
   /**
    *      sget    -       find or create a superblock
    *      @type:  filesystem type superblock should belong to
    *      @test:  comparison callback
    *      @set:   setup callback
    *      @flags: mount flags
    *      @data:  argument to each of them
    */
   struct super_block *sget(struct file_system_type *type,
                           int (*test)(struct super_block *,void *),
                           int (*set)(struct super_block *,void *),
                           int flags,
                           void *data)
   {
           struct super_block *s = NULL;
           struct hlist_node *node;
           struct super_block *old;
           int err;
   
   retry:
           spin_lock(&sb_lock);
           if (test) {
                   hlist_for_each_entry(old, node, &type->fs_supers, s_instances) {
                           if (!test(old, data))
                                   continue;
                           if (!grab_super(old))
                                   goto retry;
                           if (s) {
                                   up_write(&s->s_umount);
                                   destroy_super(s);
                                   s = NULL;
                           }
                           down_write(&old->s_umount);
                           if (unlikely(!(old->s_flags & MS_BORN))) {
                                   deactivate_locked_super(old);
                                   goto retry;
                           }
                           return old;
                   }
           }
           if (!s) {
                   spin_unlock(&sb_lock);
                   s = alloc_super(type, flags);
                   if (!s)
                           return ERR_PTR(-ENOMEM);
                   goto retry;
           }
                   
           err = set(s, data);
           if (err) {
                   spin_unlock(&sb_lock);
                   up_write(&s->s_umount);
                   destroy_super(s);
                   return ERR_PTR(err);
           }
           s->s_type = type;
           strlcpy(s->s_id, type->name, sizeof(s->s_id));
           list_add_tail(&s->s_list, &super_blocks);
           hlist_add_head(&s->s_instances, &type->fs_supers);
           spin_unlock(&sb_lock);
           get_filesystem(type);
           register_shrinker(&s->s_shrink);
           return s;
   }
   
   EXPORT_SYMBOL(sget);
   
   void drop_super(struct super_block *sb)
   {
           up_read(&sb->s_umount);
           put_super(sb);
   }
   
   EXPORT_SYMBOL(drop_super);
   
   /**
    *      iterate_supers - call function for all active superblocks
    *      @f: function to call
    *      @arg: argument to pass to it
    *
    *      Scans the superblock list and calls given function, passing it
    *      locked superblock and given argument.
    */
   void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
   {
           struct super_block *sb, *p = NULL;
   
           spin_lock(&sb_lock);
           list_for_each_entry(sb, &super_blocks, s_list) {
                   if (hlist_unhashed(&sb->s_instances))
                           continue;
                   sb->s_count++;
                   spin_unlock(&sb_lock);
   
                   down_read(&sb->s_umount);
                   if (sb->s_root && (sb->s_flags & MS_BORN))
                           f(sb, arg);
                   up_read(&sb->s_umount);
   
                   spin_lock(&sb_lock);
                   if (p)
                           __put_super(p);
                   p = sb;
           }
           if (p)
                   __put_super(p);
           spin_unlock(&sb_lock);
   }
   
   /**
    *      iterate_supers_type - call function for superblocks of given type
    *      @type: fs type
    *      @f: function to call
    *      @arg: argument to pass to it
    *
    *      Scans the superblock list and calls given function, passing it
    *      locked superblock and given argument.
    */
   void iterate_supers_type(struct file_system_type *type,
           void (*f)(struct super_block *, void *), void *arg)
   {
           struct super_block *sb, *p = NULL;
           struct hlist_node *node;
   
           spin_lock(&sb_lock);
           hlist_for_each_entry(sb, node, &type->fs_supers, s_instances) {
                   sb->s_count++;
                   spin_unlock(&sb_lock);
   
                   down_read(&sb->s_umount);
                   if (sb->s_root && (sb->s_flags & MS_BORN))
                           f(sb, arg);
                   up_read(&sb->s_umount);
   
                   spin_lock(&sb_lock);
                   if (p)
                           __put_super(p);
                   p = sb;
           }
           if (p)
                   __put_super(p);
           spin_unlock(&sb_lock);
   }
   
   EXPORT_SYMBOL(iterate_supers_type);
   
   /**
    *      get_super - get the superblock of a device
    *      @bdev: device to get the superblock for
    *      
    *      Scans the superblock list and finds the superblock of the file system
    *      mounted on the device given. %NULL is returned if no match is found.
    */
   
   struct super_block *get_super(struct block_device *bdev)
   {
           struct super_block *sb;
   
           if (!bdev)
                   return NULL;
   
           spin_lock(&sb_lock);
   rescan:
           list_for_each_entry(sb, &super_blocks, s_list) {
                   if (hlist_unhashed(&sb->s_instances))
                           continue;
                   if (sb->s_bdev == bdev) {
                           sb->s_count++;
                           spin_unlock(&sb_lock);
                           down_read(&sb->s_umount);
                           /* still alive? */
                           if (sb->s_root && (sb->s_flags & MS_BORN))
                                   return sb;
                           up_read(&sb->s_umount);
                           /* nope, got unmounted */
                           spin_lock(&sb_lock);
                           __put_super(sb);
                           goto rescan;
                   }
           }
           spin_unlock(&sb_lock);
           return NULL;
   }
   
   EXPORT_SYMBOL(get_super);
   
   /**
    *      get_super_thawed - get thawed superblock of a device
    *      @bdev: device to get the superblock for
    *
    *      Scans the superblock list and finds the superblock of the file system
    *      mounted on the device. The superblock is returned once it is thawed
    *      (or immediately if it was not frozen). %NULL is returned if no match
    *      is found.
    */
   struct super_block *get_super_thawed(struct block_device *bdev)
   {
           while (1) {
                   struct super_block *s = get_super(bdev);
                   if (!s || s->s_writers.frozen == SB_UNFROZEN)
                           return s;
                   up_read(&s->s_umount);
                   wait_event(s->s_writers.wait_unfrozen,
                              s->s_writers.frozen == SB_UNFROZEN);
                   put_super(s);
           }
   }
   EXPORT_SYMBOL(get_super_thawed);
   
   /**
    * get_active_super - get an active reference to the superblock of a device
    * @bdev: device to get the superblock for
    *
    * Scans the superblock list and finds the superblock of the file system
    * mounted on the device given.  Returns the superblock with an active
    * reference or %NULL if none was found.
    */
   struct super_block *get_active_super(struct block_device *bdev)
   {
           struct super_block *sb;
   
           if (!bdev)
                   return NULL;
   
   restart:
           spin_lock(&sb_lock);
           list_for_each_entry(sb, &super_blocks, s_list) {
                   if (hlist_unhashed(&sb->s_instances))
                           continue;
                   if (sb->s_bdev == bdev) {
                           if (grab_super(sb)) /* drops sb_lock */
                                   return sb;
                           else
                                   goto restart;
                   }
           }
           spin_unlock(&sb_lock);
           return NULL;
   }
    
   struct super_block *user_get_super(dev_t dev)
   {
           struct super_block *sb;
   
           spin_lock(&sb_lock);
   rescan:
           list_for_each_entry(sb, &super_blocks, s_list) {
                   if (hlist_unhashed(&sb->s_instances))
                           continue;
                   if (sb->s_dev ==  dev) {
                           sb->s_count++;
                           spin_unlock(&sb_lock);
                           down_read(&sb->s_umount);
                           /* still alive? */
                           if (sb->s_root && (sb->s_flags & MS_BORN))
                                   return sb;
                           up_read(&sb->s_umount);
                           /* nope, got unmounted */
                           spin_lock(&sb_lock);
                           __put_super(sb);
                           goto rescan;
                   }
           }
           spin_unlock(&sb_lock);
           return NULL;
   }
   
   /**
    *      do_remount_sb - asks filesystem to change mount options.
    *      @sb:    superblock in question
    *      @flags: numeric part of options
    *      @data:  the rest of options
    *      @force: whether or not to force the change
    *
    *      Alters the mount options of a mounted file system.
    */
   int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
   {
           int retval;
           int remount_ro;
   
           if (sb->s_writers.frozen != SB_UNFROZEN)
                   return -EBUSY;
   
   #ifdef CONFIG_BLOCK
           if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
                   return -EACCES;
   #endif
   
           if (flags & MS_RDONLY)
                   acct_auto_close(sb);
           shrink_dcache_sb(sb);
           sync_filesystem(sb);
   
           remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);
   
           /* If we are remounting RDONLY and current sb is read/write,
              make sure there are no rw files opened */
           if (remount_ro) {
                   if (force) {
                           mark_files_ro(sb);
                   } else {
                           retval = sb_prepare_remount_readonly(sb);
                           if (retval)
                                   return retval;
                   }
           }
   
           if (sb->s_op->remount_fs) {
                   retval = sb->s_op->remount_fs(sb, &flags, data);
                   if (retval) {
                           if (!force)
                                   goto cancel_readonly;
                           /* If forced remount, go ahead despite any errors */
                           WARN(1, "forced remount of a %s fs returned %i\n",
                                sb->s_type->name, retval);
                   }
           }
           sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
           /* Needs to be ordered wrt mnt_is_readonly() */
           smp_wmb();
           sb->s_readonly_remount = 0;
   
           /*
            * Some filesystems modify their metadata via some other path than the
            * bdev buffer cache (eg. use a private mapping, or directories in
            * pagecache, etc). Also file data modifications go via their own
            * mappings. So If we try to mount readonly then copy the filesystem
            * from bdev, we could get stale data, so invalidate it to give a best
            * effort at coherency.
            */
           if (remount_ro && sb->s_bdev)
                   invalidate_bdev(sb->s_bdev);
           return 0;
   
   cancel_readonly:
           sb->s_readonly_remount = 0;
           return retval;
   }
   
   static void do_emergency_remount(struct work_struct *work)
   {
           struct super_block *sb, *p = NULL;
   
           spin_lock(&sb_lock);
           list_for_each_entry(sb, &super_blocks, s_list) {
                   if (hlist_unhashed(&sb->s_instances))
                           continue;
                   sb->s_count++;
                   spin_unlock(&sb_lock);
                   down_write(&sb->s_umount);
                   if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
                       !(sb->s_flags & MS_RDONLY)) {
                           /*
                            * What lock protects sb->s_flags??
                            */
                           do_remount_sb(sb, MS_RDONLY, NULL, 1);
                   }
                   up_write(&sb->s_umount);
                   spin_lock(&sb_lock);
                   if (p)
                           __put_super(p);
                   p = sb;
           }
           if (p)
                   __put_super(p);
           spin_unlock(&sb_lock);
           kfree(work);
           printk("Emergency Remount complete\n");
   }
   
   void emergency_remount(void)
   {
           struct work_struct *work;
   
           work = kmalloc(sizeof(*work), GFP_ATOMIC);
           if (work) {
                   INIT_WORK(work, do_emergency_remount);
                   schedule_work(work);
           }
   }
   
   /*
    * Unnamed block devices are dummy devices used by virtual
    * filesystems which don't use real block-devices.  -- jrs
    */
   
   static DEFINE_IDA(unnamed_dev_ida);
   static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
   static int unnamed_dev_start = 0; /* don't bother trying below it */
   
   int get_anon_bdev(dev_t *p)
   {
           int dev;
           int error;
   
    retry:
           if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
                   return -ENOMEM;
           spin_lock(&unnamed_dev_lock);
           error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
           if (!error)
                   unnamed_dev_start = dev + 1;
           spin_unlock(&unnamed_dev_lock);
           if (error == -EAGAIN)
                   /* We raced and lost with another CPU. */
                   goto retry;
           else if (error)
                   return -EAGAIN;
   
           if ((dev & MAX_IDR_MASK) == (1 << MINORBITS)) {
                   spin_lock(&unnamed_dev_lock);
                   ida_remove(&unnamed_dev_ida, dev);
                   if (unnamed_dev_start > dev)
                           unnamed_dev_start = dev;
                   spin_unlock(&unnamed_dev_lock);
                   return -EMFILE;
           }
           *p = MKDEV(0, dev & MINORMASK);
           return 0;
   }
   EXPORT_SYMBOL(get_anon_bdev);
   
   void free_anon_bdev(dev_t dev)
   {
           int slot = MINOR(dev);
           spin_lock(&unnamed_dev_lock);
           ida_remove(&unnamed_dev_ida, slot);
           if (slot < unnamed_dev_start)
                   unnamed_dev_start = slot;
           spin_unlock(&unnamed_dev_lock);
   }
   EXPORT_SYMBOL(free_anon_bdev);
   
   int set_anon_super(struct super_block *s, void *data)
   {
           int error = get_anon_bdev(&s->s_dev);
           if (!error)
                   s->s_bdi = &noop_backing_dev_info;
           return error;
   }
   
   EXPORT_SYMBOL(set_anon_super);
   
   void kill_anon_super(struct super_block *sb)
   {
           dev_t dev = sb->s_dev;
           generic_shutdown_super(sb);
           free_anon_bdev(dev);
   }
   
   EXPORT_SYMBOL(kill_anon_super);
   
   void kill_litter_super(struct super_block *sb)
   {
           if (sb->s_root)
                   d_genocide(sb->s_root);
           kill_anon_super(sb);
   }
   
   EXPORT_SYMBOL(kill_litter_super);
   
   static int ns_test_super(struct super_block *sb, void *data)
   {
           return sb->s_fs_info == data;
   }
   
   static int ns_set_super(struct super_block *sb, void *data)
   {
           sb->s_fs_info = data;
           return set_anon_super(sb, NULL);
   }
   
   struct dentry *mount_ns(struct file_system_type *fs_type, int flags,
           void *data, int (*fill_super)(struct super_block *, void *, int))
   {
           struct super_block *sb;
   
           sb = sget(fs_type, ns_test_super, ns_set_super, flags, data);
           if (IS_ERR(sb))
                   return ERR_CAST(sb);
   
           if (!sb->s_root) {
                   int err;
                   err = fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
                   if (err) {
                           deactivate_locked_super(sb);
                           return ERR_PTR(err);
                   }
   
                   sb->s_flags |= MS_ACTIVE;
           }
   
           return dget(sb->s_root);
   }
   
   EXPORT_SYMBOL(mount_ns);
   
   #ifdef CONFIG_BLOCK
   static int set_bdev_super(struct super_block *s, void *data)
   {
           s->s_bdev = data;
           s->s_dev = s->s_bdev->bd_dev;
   
           /*
            * We set the bdi here to the queue backing, file systems can
            * overwrite this in ->fill_super()
            */
           s->s_bdi = &bdev_get_queue(s->s_bdev)->backing_dev_info;
           return 0;
   }
   
   static int test_bdev_super(struct super_block *s, void *data)
   {
           return (void *)s->s_bdev == data;
   }
   
   struct dentry *mount_bdev(struct file_system_type *fs_type,
           int flags, const char *dev_name, void *data,
           int (*fill_super)(struct super_block *, void *, int))
   {
           struct block_device *bdev;
           struct super_block *s;
           fmode_t mode = FMODE_READ | FMODE_EXCL;
           int error = 0;
   
           if (!(flags & MS_RDONLY))
                   mode |= FMODE_WRITE;
   
           bdev = blkdev_get_by_path(dev_name, mode, fs_type);
           if (IS_ERR(bdev))
                   return ERR_CAST(bdev);
   
           /*
            * once the super is inserted into the list by sget, s_umount
            * will protect the lockfs code from trying to start a snapshot
            * while we are mounting
            */
           mutex_lock(&bdev->bd_fsfreeze_mutex);
           if (bdev->bd_fsfreeze_count > 0) {
                   mutex_unlock(&bdev->bd_fsfreeze_mutex);
                   error = -EBUSY;
                   goto error_bdev;
           }
           s = sget(fs_type, test_bdev_super, set_bdev_super, flags | MS_NOSEC,
                    bdev);
           mutex_unlock(&bdev->bd_fsfreeze_mutex);
           if (IS_ERR(s))
                   goto error_s;
   
           if (s->s_root) {
                   if ((flags ^ s->s_flags) & MS_RDONLY) {
                           deactivate_locked_super(s);
                           error = -EBUSY;
                           goto error_bdev;
                   }
   
                   /*
                    * s_umount nests inside bd_mutex during
                    * __invalidate_device().  blkdev_put() acquires
                    * bd_mutex and can't be called under s_umount.  Drop
                    * s_umount temporarily.  This is safe as we're
                    * holding an active reference.
                    */
                   up_write(&s->s_umount);
                  blkdev_put(bdev, mode);
                  down_write(&s->s_umount);
          } else {
                  char b[BDEVNAME_SIZE];
  
                  s->s_mode = mode;
                  strlcpy(s->s_id, bdevname(bdev, b), sizeof(s->s_id));
                  sb_set_blocksize(s, block_size(bdev));
                  error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
                  if (error) {
                          deactivate_locked_super(s);
                          goto error;
                  }
  
                  s->s_flags |= MS_ACTIVE;
                  bdev->bd_super = s;
          }
  
          return dget(s->s_root);
  
  error_s:
          error = PTR_ERR(s);
  error_bdev:
          blkdev_put(bdev, mode);
  error:
          return ERR_PTR(error);
  }
  EXPORT_SYMBOL(mount_bdev);
  
  void kill_block_super(struct super_block *sb)
  {
          struct block_device *bdev = sb->s_bdev;
          fmode_t mode = sb->s_mode;
  
          bdev->bd_super = NULL;
          generic_shutdown_super(sb);
          sync_blockdev(bdev);
          WARN_ON_ONCE(!(mode & FMODE_EXCL));
          blkdev_put(bdev, mode | FMODE_EXCL);
  }
  
  EXPORT_SYMBOL(kill_block_super);
  #endif
  
  struct dentry *mount_nodev(struct file_system_type *fs_type,
          int flags, void *data,
          int (*fill_super)(struct super_block *, void *, int))
  {
          int error;
          struct super_block *s = sget(fs_type, NULL, set_anon_super, flags, NULL);
  
          if (IS_ERR(s))
                  return ERR_CAST(s);
  
          error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
          if (error) {
                  deactivate_locked_super(s);
                  return ERR_PTR(error);
          }
          s->s_flags |= MS_ACTIVE;
          return dget(s->s_root);
  }
  EXPORT_SYMBOL(mount_nodev);
  
  static int compare_single(struct super_block *s, void *p)
  {
          return 1;
  }
  
  struct dentry *mount_single(struct file_system_type *fs_type,
          int flags, void *data,
          int (*fill_super)(struct super_block *, void *, int))
  {
          struct super_block *s;
          int error;
  
          s = sget(fs_type, compare_single, set_anon_super, flags, NULL);
          if (IS_ERR(s))
                  return ERR_CAST(s);
          if (!s->s_root) {
                  error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
                  if (error) {
                          deactivate_locked_super(s);
                          return ERR_PTR(error);
                  }
                  s->s_flags |= MS_ACTIVE;
          } else {
                  do_remount_sb(s, flags, data, 0);
          }
          return dget(s->s_root);
  }
  EXPORT_SYMBOL(mount_single);
  
  struct dentry *
  mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
  {
          struct dentry *root;
          struct super_block *sb;
          char *secdata = NULL;
          int error = -ENOMEM;
  
          if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
                  secdata = alloc_secdata();
                  if (!secdata)
                          goto out;
  
                  error = security_sb_copy_data(data, secdata);
                  if (error)
                          goto out_free_secdata;
          }
  
          root = type->mount(type, flags, name, data);
          if (IS_ERR(root)) {
                  error = PTR_ERR(root);
                  goto out_free_secdata;
          }
          sb = root->d_sb;
          BUG_ON(!sb);
          WARN_ON(!sb->s_bdi);
          WARN_ON(sb->s_bdi == &default_backing_dev_info);
          sb->s_flags |= MS_BORN;
  
          error = security_sb_kern_mount(sb, flags, secdata);
          if (error)
                  goto out_sb;
  
          /*
           * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
           * but s_maxbytes was an unsigned long long for many releases. Throw
           * this warning for a little while to try and catch filesystems that
           * violate this rule.
           */
          WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
                  "negative value (%lld)\n", type->name, sb->s_maxbytes);
  
          up_write(&sb->s_umount);
          free_secdata(secdata);
          return root;
  out_sb:
          dput(root);
          deactivate_locked_super(sb);
  out_free_secdata:
          free_secdata(secdata);
  out:
          return ERR_PTR(error);
  }
  
  /*
   * This is an internal function, please use sb_end_{write,pagefault,intwrite}
   * instead.
   */
  void __sb_end_write(struct super_block *sb, int level)
  {
          percpu_counter_dec(&sb->s_writers.counter[level-1]);
          /*
           * Make sure s_writers are updated before we wake up waiters in
           * freeze_super().
           */
          smp_mb();
          if (waitqueue_active(&sb->s_writers.wait))
                  wake_up(&sb->s_writers.wait);
          rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
  }
  EXPORT_SYMBOL(__sb_end_write);
  
  #ifdef CONFIG_LOCKDEP
  /*
   * We want lockdep to tell us about possible deadlocks with freezing but
   * it's it bit tricky to properly instrument it. Getting a freeze protection
   * works as getting a read lock but there are subtle problems. XFS for example
   * gets freeze protection on internal level twice in some cases, which is OK
   * only because we already hold a freeze protection also on higher level. Due
   * to these cases we have to tell lockdep we are doing trylock when we
   * already hold a freeze protection for a higher freeze level.
   */
  static void acquire_freeze_lock(struct super_block *sb, int level, bool trylock,
                                  unsigned long ip)
  {
          int i;
  
          if (!trylock) {
                  for (i = 0; i < level - 1; i++)
                          if (lock_is_held(&sb->s_writers.lock_map[i])) {
                                  trylock = true;
                                  break;
                          }
          }
          rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, trylock, ip);
  }
  #endif
  
  /*
   * This is an internal function, please use sb_start_{write,pagefault,intwrite}
   * instead.
   */
  int __sb_start_write(struct super_block *sb, int level, bool wait)
  {
  retry:
          if (unlikely(sb->s_writers.frozen >= level)) {
                  if (!wait)
                          return 0;
                  wait_event(sb->s_writers.wait_unfrozen,
                             sb->s_writers.frozen < level);
          }
  
  #ifdef CONFIG_LOCKDEP
          acquire_freeze_lock(sb, level, !wait, _RET_IP_);
  #endif
          percpu_counter_inc(&sb->s_writers.counter[level-1]);
          /*
           * Make sure counter is updated before we check for frozen.
           * freeze_super() first sets frozen and then checks the counter.
           */
          smp_mb();
          if (unlikely(sb->s_writers.frozen >= level)) {
                  __sb_end_write(sb, level);
                  goto retry;
          }
          return 1;
  }
  EXPORT_SYMBOL(__sb_start_write);
  
  /**
   * sb_wait_write - wait until all writers to given file system finish
   * @sb: the super for which we wait
   * @level: type of writers we wait for (normal vs page fault)
   *
   * This function waits until there are no writers of given type to given file
   * system. Caller of this function should make sure there can be no new writers
   * of type @level before calling this function. Otherwise this function can
   * livelock.
   */
  static void sb_wait_write(struct super_block *sb, int level)
  {
          s64 writers;
  
          /*
           * We just cycle-through lockdep here so that it does not complain
           * about returning with lock to userspace
           */
          rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
          rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
  
          do {
                  DEFINE_WAIT(wait);
  
                  /*
                   * We use a barrier in prepare_to_wait() to separate setting
                   * of frozen and checking of the counter
                   */
                  prepare_to_wait(&sb->s_writers.wait, &wait,
                                  TASK_UNINTERRUPTIBLE);
  
                  writers = percpu_counter_sum(&sb->s_writers.counter[level-1]);
                  if (writers)
                          schedule();
  
                  finish_wait(&sb->s_writers.wait, &wait);
          } while (writers);
  }
  
  /**
   * freeze_super - lock the filesystem and force it into a consistent state
   * @sb: the super to lock
   *
   * Syncs the super to make sure the filesystem is consistent and calls the fs's
   * freeze_fs.  Subsequent calls to this without first thawing the fs will return
   * -EBUSY.
   *
   * During this function, sb->s_writers.frozen goes through these values:
   *
   * SB_UNFROZEN: File system is normal, all writes progress as usual.
   *
   * SB_FREEZE_WRITE: The file system is in the process of being frozen.  New
   * writes should be blocked, though page faults are still allowed. We wait for
   * all writes to complete and then proceed to the next stage.
   *
   * SB_FREEZE_PAGEFAULT: Freezing continues. Now also page faults are blocked
   * but internal fs threads can still modify the filesystem (although they
   * should not dirty new pages or inodes), writeback can run etc. After waiting
   * for all running page faults we sync the filesystem which will clean all
   * dirty pages and inodes (no new dirty pages or inodes can be created when
   * sync is running).
   *
   * SB_FREEZE_FS: The file system is frozen. Now all internal sources of fs
   * modification are blocked (e.g. XFS preallocation truncation on inode
   * reclaim). This is usually implemented by blocking new transactions for
   * filesystems that have them and need this additional guard. After all
   * internal writers are finished we call ->freeze_fs() to finish filesystem
   * freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
   * mostly auxiliary for filesystems to verify they do not modify frozen fs.
   *
   * sb->s_writers.frozen is protected by sb->s_umount.
   */
  int freeze_super(struct super_block *sb)
  {
          int ret;
  
          atomic_inc(&sb->s_active);
          down_write(&sb->s_umount);
          if (sb->s_writers.frozen != SB_UNFROZEN) {
                  deactivate_locked_super(sb);
                  return -EBUSY;
          }
  
          if (!(sb->s_flags & MS_BORN)) {
                  up_write(&sb->s_umount);
                  return 0;       /* sic - it's "nothing to do" */
          }
  
          if (sb->s_flags & MS_RDONLY) {
                  /* Nothing to do really... */
                  sb->s_writers.frozen = SB_FREEZE_COMPLETE;
                  up_write(&sb->s_umount);
                  return 0;
          }
  
          /* From now on, no new normal writers can start */
          sb->s_writers.frozen = SB_FREEZE_WRITE;
          smp_wmb();
  
          /* Release s_umount to preserve sb_start_write -> s_umount ordering */
          up_write(&sb->s_umount);
  
          sb_wait_write(sb, SB_FREEZE_WRITE);
  
          /* Now we go and block page faults... */
          down_write(&sb->s_umount);
          sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
          smp_wmb();
  
          sb_wait_write(sb, SB_FREEZE_PAGEFAULT);
  
          /* All writers are done so after syncing there won't be dirty data */
          sync_filesystem(sb);
  
          /* Now wait for internal filesystem counter */
          sb->s_writers.frozen = SB_FREEZE_FS;
          smp_wmb();
          sb_wait_write(sb, SB_FREEZE_FS);
  
          if (sb->s_op->freeze_fs) {
                  ret = sb->s_op->freeze_fs(sb);
                  if (ret) {
                          printk(KERN_ERR
                                  "VFS:Filesystem freeze failed\n");
                          sb->s_writers.frozen = SB_UNFROZEN;
                          smp_wmb();
                          wake_up(&sb->s_writers.wait_unfrozen);
                          deactivate_locked_super(sb);
                          return ret;
                  }
          }
          /*
           * This is just for debugging purposes so that fs can warn if it
           * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
           */
          sb->s_writers.frozen = SB_FREEZE_COMPLETE;
          up_write(&sb->s_umount);
          return 0;
  }
  EXPORT_SYMBOL(freeze_super);
  
  /**
   * thaw_super -- unlock filesystem
   * @sb: the super to thaw
   *
   * Unlocks the filesystem and marks it writeable again after freeze_super().
   */
  int thaw_super(struct super_block *sb)
  {
          int error;
  
          down_write(&sb->s_umount);
          if (sb->s_writers.frozen == SB_UNFROZEN) {
                  up_write(&sb->s_umount);
                  return -EINVAL;
          }
  
          if (sb->s_flags & MS_RDONLY)
                  goto out;
  
          if (sb->s_op->unfreeze_fs) {
                  error = sb->s_op->unfreeze_fs(sb);
                  if (error) {
                          printk(KERN_ERR
                                  "VFS:Filesystem thaw failed\n");
                          up_write(&sb->s_umount);
                          return error;
                  }
          }
  
  out:
          sb->s_writers.frozen = SB_UNFROZEN;
          smp_wmb();
          wake_up(&sb->s_writers.wait_unfrozen);
          deactivate_locked_super(sb);
  
          return 0;
  }
  EXPORT_SYMBOL(thaw_super);

Cache object: f30d20d66a8ed84f516b20ba79bfc32d