The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/lib/percpu-rwsem.c

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    1 #include <linux/atomic.h>
    2 #include <linux/rwsem.h>
    3 #include <linux/percpu.h>
    4 #include <linux/wait.h>
    5 #include <linux/lockdep.h>
    6 #include <linux/percpu-rwsem.h>
    7 #include <linux/rcupdate.h>
    8 #include <linux/sched.h>
    9 #include <linux/errno.h>
   10 
   11 int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
   12                         const char *name, struct lock_class_key *rwsem_key)
   13 {
   14         brw->fast_read_ctr = alloc_percpu(int);
   15         if (unlikely(!brw->fast_read_ctr))
   16                 return -ENOMEM;
   17 
   18         /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
   19         __init_rwsem(&brw->rw_sem, name, rwsem_key);
   20         atomic_set(&brw->write_ctr, 0);
   21         atomic_set(&brw->slow_read_ctr, 0);
   22         init_waitqueue_head(&brw->write_waitq);
   23         return 0;
   24 }
   25 
   26 void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
   27 {
   28         free_percpu(brw->fast_read_ctr);
   29         brw->fast_read_ctr = NULL; /* catch use after free bugs */
   30 }
   31 
   32 /*
   33  * This is the fast-path for down_read/up_read, it only needs to ensure
   34  * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
   35  * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
   36  * serialize with the preempt-disabled section below.
   37  *
   38  * The nontrivial part is that we should guarantee acquire/release semantics
   39  * in case when
   40  *
   41  *      R_W: down_write() comes after up_read(), the writer should see all
   42  *           changes done by the reader
   43  * or
   44  *      W_R: down_read() comes after up_write(), the reader should see all
   45  *           changes done by the writer
   46  *
   47  * If this helper fails the callers rely on the normal rw_semaphore and
   48  * atomic_dec_and_test(), so in this case we have the necessary barriers.
   49  *
   50  * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
   51  * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
   52  * reader inside the critical section. See the comments in down_write and
   53  * up_write below.
   54  */
   55 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
   56 {
   57         bool success = false;
   58 
   59         preempt_disable();
   60         if (likely(!atomic_read(&brw->write_ctr))) {
   61                 __this_cpu_add(*brw->fast_read_ctr, val);
   62                 success = true;
   63         }
   64         preempt_enable();
   65 
   66         return success;
   67 }
   68 
   69 /*
   70  * Like the normal down_read() this is not recursive, the writer can
   71  * come after the first percpu_down_read() and create the deadlock.
   72  *
   73  * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
   74  * percpu_up_read() does rwsem_release(). This pairs with the usage
   75  * of ->rw_sem in percpu_down/up_write().
   76  */
   77 void percpu_down_read(struct percpu_rw_semaphore *brw)
   78 {
   79         might_sleep();
   80         if (likely(update_fast_ctr(brw, +1))) {
   81                 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
   82                 return;
   83         }
   84 
   85         down_read(&brw->rw_sem);
   86         atomic_inc(&brw->slow_read_ctr);
   87         /* avoid up_read()->rwsem_release() */
   88         __up_read(&brw->rw_sem);
   89 }
   90 
   91 void percpu_up_read(struct percpu_rw_semaphore *brw)
   92 {
   93         rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
   94 
   95         if (likely(update_fast_ctr(brw, -1)))
   96                 return;
   97 
   98         /* false-positive is possible but harmless */
   99         if (atomic_dec_and_test(&brw->slow_read_ctr))
  100                 wake_up_all(&brw->write_waitq);
  101 }
  102 
  103 static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
  104 {
  105         unsigned int sum = 0;
  106         int cpu;
  107 
  108         for_each_possible_cpu(cpu) {
  109                 sum += per_cpu(*brw->fast_read_ctr, cpu);
  110                 per_cpu(*brw->fast_read_ctr, cpu) = 0;
  111         }
  112 
  113         return sum;
  114 }
  115 
  116 /*
  117  * A writer increments ->write_ctr to force the readers to switch to the
  118  * slow mode, note the atomic_read() check in update_fast_ctr().
  119  *
  120  * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
  121  * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
  122  * counter it represents the number of active readers.
  123  *
  124  * Finally the writer takes ->rw_sem for writing and blocks the new readers,
  125  * then waits until the slow counter becomes zero.
  126  */
  127 void percpu_down_write(struct percpu_rw_semaphore *brw)
  128 {
  129         /* tell update_fast_ctr() there is a pending writer */
  130         atomic_inc(&brw->write_ctr);
  131         /*
  132          * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
  133          *    so that update_fast_ctr() can't succeed.
  134          *
  135          * 2. Ensures we see the result of every previous this_cpu_add() in
  136          *    update_fast_ctr().
  137          *
  138          * 3. Ensures that if any reader has exited its critical section via
  139          *    fast-path, it executes a full memory barrier before we return.
  140          *    See R_W case in the comment above update_fast_ctr().
  141          */
  142         synchronize_sched_expedited();
  143 
  144         /* exclude other writers, and block the new readers completely */
  145         down_write(&brw->rw_sem);
  146 
  147         /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
  148         atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
  149 
  150         /* wait for all readers to complete their percpu_up_read() */
  151         wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
  152 }
  153 
  154 void percpu_up_write(struct percpu_rw_semaphore *brw)
  155 {
  156         /* release the lock, but the readers can't use the fast-path */
  157         up_write(&brw->rw_sem);
  158         /*
  159          * Insert the barrier before the next fast-path in down_read,
  160          * see W_R case in the comment above update_fast_ctr().
  161          */
  162         synchronize_sched_expedited();
  163         /* the last writer unblocks update_fast_ctr() */
  164         atomic_dec(&brw->write_ctr);
  165 }

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