lib/percpu-rwsem.c

   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 }