kernel/locking/percpu-rwsem.c

   1 #include <linux/atomic.h>
   2 #include <linux/rwsem.h>
   3 #include <linux/percpu.h>
   4 #include <linux/lockdep.h>
   5 #include <linux/percpu-rwsem.h>
   6 #include <linux/rcupdate.h>
   7 #include <linux/sched.h>
   8 #include <linux/errno.h>
   9
  10 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
  11                         const char *name, struct lock_class_key *rwsem_key)
  12 {
  13         sem->read_count = alloc_percpu(int);
  14         if (unlikely(!sem->read_count))
  15                 return -ENOMEM;
  16
  17         /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
  18         rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
  19         __init_rwsem(&sem->rw_sem, name, rwsem_key);
  20         rcuwait_init(&sem->writer);
  21         sem->readers_block = 0;
  22         return 0;
  23 }
  24 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
  25
  26 void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
  27 {
  28         /*
  29          * XXX: temporary kludge. The error path in alloc_super()
  30          * assumes that percpu_free_rwsem() is safe after kzalloc().
  31          */
  32         if (!sem->read_count)
  33                 return;
  34
  35         rcu_sync_dtor(&sem->rss);
  36         free_percpu(sem->read_count);
  37         sem->read_count = NULL; /* catch use after free bugs */
  38 }
  39 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
  40
  41 int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
  42 {
  43         /*
  44          * Due to having preemption disabled the decrement happens on
  45          * the same CPU as the increment, avoiding the
  46          * increment-on-one-CPU-and-decrement-on-another problem.
  47          *
  48          * If the reader misses the writer's assignment of readers_block, then
  49          * the writer is guaranteed to see the reader's increment.
  50          *
  51          * Conversely, any readers that increment their sem->read_count after
  52          * the writer looks are guaranteed to see the readers_block value,
  53          * which in turn means that they are guaranteed to immediately
  54          * decrement their sem->read_count, so that it doesn't matter that the
  55          * writer missed them.
  56          */
  57
  58         smp_mb(); /* A matches D */
  59
  60         /*
  61          * If !readers_block the critical section starts here, matched by the
  62          * release in percpu_up_write().
  63          */
  64         if (likely(!smp_load_acquire(&sem->readers_block)))
  65                 return 1;
  66
  67         /*
  68          * Per the above comment; we still have preemption disabled and
  69          * will thus decrement on the same CPU as we incremented.
  70          */
  71         __percpu_up_read(sem);
  72
  73         if (try)
  74                 return 0;
  75
  76         /*
  77          * We either call schedule() in the wait, or we'll fall through
  78          * and reschedule on the preempt_enable() in percpu_down_read().
  79          */
  80         preempt_enable_no_resched();
  81
  82         /*
  83          * Avoid lockdep for the down/up_read() we already have them.
  84          */
  85         __down_read(&sem->rw_sem);
  86         this_cpu_inc(*sem->read_count);
  87         __up_read(&sem->rw_sem);
  88
  89         preempt_disable();
  90         return 1;
  91 }
  92 EXPORT_SYMBOL_GPL(__percpu_down_read);
  93
  94 void __percpu_up_read(struct percpu_rw_semaphore *sem)
  95 {
  96         smp_mb(); /* B matches C */
  97         /*
  98          * In other words, if they see our decrement (presumably to aggregate
  99          * zero, as that is the only time it matters) they will also see our
 100          * critical section.
 101          */
 102         __this_cpu_dec(*sem->read_count);
 103
 104         /* Prod writer to recheck readers_active */
 105         rcuwait_wake_up(&sem->writer);
 106 }
 107 EXPORT_SYMBOL_GPL(__percpu_up_read);
 108
 109 #define per_cpu_sum(var)                                                \
 110 ({                                                                      \
 111         typeof(var) __sum = 0;                                          \
 112         int cpu;                                                        \
 113         compiletime_assert_atomic_type(__sum);                          \
 114         for_each_possible_cpu(cpu)                                      \
 115                 __sum += per_cpu(var, cpu);                             \
 116         __sum;                                                          \
 117 })
 118
 119 /*
 120  * Return true if the modular sum of the sem->read_count per-CPU variable is
 121  * zero.  If this sum is zero, then it is stable due to the fact that if any
 122  * newly arriving readers increment a given counter, they will immediately
 123  * decrement that same counter.
 124  */
 125 static bool readers_active_check(struct percpu_rw_semaphore *sem)
 126 {
 127         if (per_cpu_sum(*sem->read_count) != 0)
 128                 return false;
 129
 130         /*
 131          * If we observed the decrement; ensure we see the entire critical
 132          * section.
 133          */
 134
 135         smp_mb(); /* C matches B */
 136
 137         return true;
 138 }
 139
 140 void percpu_down_write(struct percpu_rw_semaphore *sem)
 141 {
 142         /* Notify readers to take the slow path. */
 143         rcu_sync_enter(&sem->rss);
 144
 145         down_write(&sem->rw_sem);
 146
 147         /*
 148          * Notify new readers to block; up until now, and thus throughout the
 149          * longish rcu_sync_enter() above, new readers could still come in.
 150          */
 151         WRITE_ONCE(sem->readers_block, 1);
 152
 153         smp_mb(); /* D matches A */
 154
 155         /*
 156          * If they don't see our writer of readers_block, then we are
 157          * guaranteed to see their sem->read_count increment, and therefore
 158          * will wait for them.
 159          */
 160
 161         /* Wait for all now active readers to complete. */
 162         rcuwait_wait_event(&sem->writer, readers_active_check(sem));
 163 }
 164 EXPORT_SYMBOL_GPL(percpu_down_write);
 165
 166 void percpu_up_write(struct percpu_rw_semaphore *sem)
 167 {
 168         /*
 169          * Signal the writer is done, no fast path yet.
 170          *
 171          * One reason that we cannot just immediately flip to readers_fast is
 172          * that new readers might fail to see the results of this writer's
 173          * critical section.
 174          *
 175          * Therefore we force it through the slow path which guarantees an
 176          * acquire and thereby guarantees the critical section's consistency.
 177          */
 178         smp_store_release(&sem->readers_block, 0);
 179
 180         /*
 181          * Release the write lock, this will allow readers back in the game.
 182          */
 183         up_write(&sem->rw_sem);
 184
 185         /*
 186          * Once this completes (at least one RCU-sched grace period hence) the
 187          * reader fast path will be available again. Safe to use outside the
 188          * exclusive write lock because its counting.
 189          */
 190         rcu_sync_exit(&sem->rss);
 191 }
 192 EXPORT_SYMBOL_GPL(percpu_up_write);