The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/robust-futexes.txt

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    1 Started by: Ingo Molnar <mingo@redhat.com>
    2 
    3 Background
    4 ----------
    5 
    6 what are robust futexes? To answer that, we first need to understand
    7 what futexes are: normal futexes are special types of locks that in the
    8 noncontended case can be acquired/released from userspace without having
    9 to enter the kernel.
   10 
   11 A futex is in essence a user-space address, e.g. a 32-bit lock variable
   12 field. If userspace notices contention (the lock is already owned and
   13 someone else wants to grab it too) then the lock is marked with a value
   14 that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
   15 syscall is used to wait for the other guy to release it. The kernel
   16 creates a 'futex queue' internally, so that it can later on match up the
   17 waiter with the waker - without them having to know about each other.
   18 When the owner thread releases the futex, it notices (via the variable
   19 value) that there were waiter(s) pending, and does the
   20 sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have
   21 taken and released the lock, the futex is again back to 'uncontended'
   22 state, and there's no in-kernel state associated with it. The kernel
   23 completely forgets that there ever was a futex at that address. This
   24 method makes futexes very lightweight and scalable.
   25 
   26 "Robustness" is about dealing with crashes while holding a lock: if a
   27 process exits prematurely while holding a pthread_mutex_t lock that is
   28 also shared with some other process (e.g. yum segfaults while holding a
   29 pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
   30 to be notified that the last owner of the lock exited in some irregular
   31 way.
   32 
   33 To solve such types of problems, "robust mutex" userspace APIs were
   34 created: pthread_mutex_lock() returns an error value if the owner exits
   35 prematurely - and the new owner can decide whether the data protected by
   36 the lock can be recovered safely.
   37 
   38 There is a big conceptual problem with futex based mutexes though: it is
   39 the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
   40 the kernel cannot help with the cleanup: if there is no 'futex queue'
   41 (and in most cases there is none, futexes being fast lightweight locks)
   42 then the kernel has no information to clean up after the held lock!
   43 Userspace has no chance to clean up after the lock either - userspace is
   44 the one that crashes, so it has no opportunity to clean up. Catch-22.
   45 
   46 In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
   47 is needed to release that futex based lock. This is one of the leading
   48 bugreports against yum.
   49 
   50 To solve this problem, the traditional approach was to extend the vma
   51 (virtual memory area descriptor) concept to have a notion of 'pending
   52 robust futexes attached to this area'. This approach requires 3 new
   53 syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
   54 FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
   55 they have a robust_head set. This approach has two fundamental problems
   56 left:
   57 
   58  - it has quite complex locking and race scenarios. The vma-based
   59    approach had been pending for years, but they are still not completely
   60    reliable.
   61 
   62  - they have to scan _every_ vma at sys_exit() time, per thread!
   63 
   64 The second disadvantage is a real killer: pthread_exit() takes around 1
   65 microsecond on Linux, but with thousands (or tens of thousands) of vmas
   66 every pthread_exit() takes a millisecond or more, also totally
   67 destroying the CPU's L1 and L2 caches!
   68 
   69 This is very much noticeable even for normal process sys_exit_group()
   70 calls: the kernel has to do the vma scanning unconditionally! (this is
   71 because the kernel has no knowledge about how many robust futexes there
   72 are to be cleaned up, because a robust futex might have been registered
   73 in another task, and the futex variable might have been simply mmap()-ed
   74 into this process's address space).
   75 
   76 This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
   77 normal kernels can turn it off, but worse than that: the overhead makes
   78 robust futexes impractical for any type of generic Linux distribution.
   79 
   80 So something had to be done.
   81 
   82 New approach to robust futexes
   83 ------------------------------
   84 
   85 At the heart of this new approach there is a per-thread private list of
   86 robust locks that userspace is holding (maintained by glibc) - which
   87 userspace list is registered with the kernel via a new syscall [this
   88 registration happens at most once per thread lifetime]. At do_exit()
   89 time, the kernel checks this user-space list: are there any robust futex
   90 locks to be cleaned up?
   91 
   92 In the common case, at do_exit() time, there is no list registered, so
   93 the cost of robust futexes is just a simple current->robust_list != NULL
   94 comparison. If the thread has registered a list, then normally the list
   95 is empty. If the thread/process crashed or terminated in some incorrect
   96 way then the list might be non-empty: in this case the kernel carefully
   97 walks the list [not trusting it], and marks all locks that are owned by
   98 this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
   99 any).
  100 
  101 The list is guaranteed to be private and per-thread at do_exit() time,
  102 so it can be accessed by the kernel in a lockless way.
  103 
  104 There is one race possible though: since adding to and removing from the
  105 list is done after the futex is acquired by glibc, there is a few
  106 instructions window for the thread (or process) to die there, leaving
  107 the futex hung. To protect against this possibility, userspace (glibc)
  108 also maintains a simple per-thread 'list_op_pending' field, to allow the
  109 kernel to clean up if the thread dies after acquiring the lock, but just
  110 before it could have added itself to the list. Glibc sets this
  111 list_op_pending field before it tries to acquire the futex, and clears
  112 it after the list-add (or list-remove) has finished.
  113 
  114 That's all that is needed - all the rest of robust-futex cleanup is done
  115 in userspace [just like with the previous patches].
  116 
  117 Ulrich Drepper has implemented the necessary glibc support for this new
  118 mechanism, which fully enables robust mutexes.
  119 
  120 Key differences of this userspace-list based approach, compared to the
  121 vma based method:
  122 
  123  - it's much, much faster: at thread exit time, there's no need to loop
  124    over every vma (!), which the VM-based method has to do. Only a very
  125    simple 'is the list empty' op is done.
  126 
  127  - no VM changes are needed - 'struct address_space' is left alone.
  128 
  129  - no registration of individual locks is needed: robust mutexes dont
  130    need any extra per-lock syscalls. Robust mutexes thus become a very
  131    lightweight primitive - so they dont force the application designer
  132    to do a hard choice between performance and robustness - robust
  133    mutexes are just as fast.
  134 
  135  - no per-lock kernel allocation happens.
  136 
  137  - no resource limits are needed.
  138 
  139  - no kernel-space recovery call (FUTEX_RECOVER) is needed.
  140 
  141  - the implementation and the locking is "obvious", and there are no
  142    interactions with the VM.
  143 
  144 Performance
  145 -----------
  146 
  147 I have benchmarked the time needed for the kernel to process a list of 1
  148 million (!) held locks, using the new method [on a 2GHz CPU]:
  149 
  150  - with FUTEX_WAIT set [contended mutex]: 130 msecs
  151  - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
  152 
  153 I have also measured an approach where glibc does the lock notification
  154 [which it currently does for !pshared robust mutexes], and that took 256
  155 msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
  156 userspace had to do.
  157 
  158 (1 million held locks are unheard of - we expect at most a handful of
  159 locks to be held at a time. Nevertheless it's nice to know that this
  160 approach scales nicely.)
  161 
  162 Implementation details
  163 ----------------------
  164 
  165 The patch adds two new syscalls: one to register the userspace list, and
  166 one to query the registered list pointer:
  167 
  168  asmlinkage long
  169  sys_set_robust_list(struct robust_list_head __user *head,
  170                      size_t len);
  171 
  172  asmlinkage long
  173  sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
  174                      size_t __user *len_ptr);
  175 
  176 List registration is very fast: the pointer is simply stored in
  177 current->robust_list. [Note that in the future, if robust futexes become
  178 widespread, we could extend sys_clone() to register a robust-list head
  179 for new threads, without the need of another syscall.]
  180 
  181 So there is virtually zero overhead for tasks not using robust futexes,
  182 and even for robust futex users, there is only one extra syscall per
  183 thread lifetime, and the cleanup operation, if it happens, is fast and
  184 straightforward. The kernel doesn't have any internal distinction between
  185 robust and normal futexes.
  186 
  187 If a futex is found to be held at exit time, the kernel sets the
  188 following bit of the futex word:
  189 
  190         #define FUTEX_OWNER_DIED        0x40000000
  191 
  192 and wakes up the next futex waiter (if any). User-space does the rest of
  193 the cleanup.
  194 
  195 Otherwise, robust futexes are acquired by glibc by putting the TID into
  196 the futex field atomically. Waiters set the FUTEX_WAITERS bit:
  197 
  198         #define FUTEX_WAITERS           0x80000000
  199 
  200 and the remaining bits are for the TID.
  201 
  202 Testing, architecture support
  203 -----------------------------
  204 
  205 i've tested the new syscalls on x86 and x86_64, and have made sure the
  206 parsing of the userspace list is robust [ ;-) ] even if the list is
  207 deliberately corrupted.
  208 
  209 i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
  210 tested the new glibc code (on x86_64 and i386), and it works for his
  211 robust-mutex testcases.
  212 
  213 All other architectures should build just fine too - but they wont have
  214 the new syscalls yet.
  215 
  216 Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
  217 inline function before writing up the syscalls (that function returns
  218 -ENOSYS right now).

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