kernel/locking/percpu-rwsem.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 #include <linux/atomic.h>
   3 #include <linux/rwsem.h>
   4 #include <linux/percpu.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 #include "rwsem.h"
  12
  13 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
  14                         const char *name, struct lock_class_key *rwsem_key)
  15 {
  16         sem->read_count = alloc_percpu(int);
  17         if (unlikely(!sem->read_count))
  18                 return -ENOMEM;
  19
  20         /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
  21         rcu_sync_init(&sem->rss);
  22         __init_rwsem(&sem->rw_sem, name, rwsem_key);
  23         rcuwait_init(&sem->writer);
  24         sem->readers_block = 0;
  25         return 0;
  26 }
  27 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
  28
  29 void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
  30 {
  31         /*
  32          * XXX: temporary kludge. The error path in alloc_super()
  33          * assumes that percpu_free_rwsem() is safe after kzalloc().
  34          */
  35         if (!sem->read_count)
  36                 return;
  37
  38         rcu_sync_dtor(&sem->rss);
  39         free_percpu(sem->read_count);
  40         sem->read_count = NULL; /* catch use after free bugs */
  41 }
  42 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
  43
  44 int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
  45 {
  46         /*
  47          * Due to having preemption disabled the decrement happens on
  48          * the same CPU as the increment, avoiding the
  49          * increment-on-one-CPU-and-decrement-on-another problem.
  50          *
  51          * If the reader misses the writer's assignment of readers_block, then
  52          * the writer is guaranteed to see the reader's increment.
  53          *
  54          * Conversely, any readers that increment their sem->read_count after
  55          * the writer looks are guaranteed to see the readers_block value,
  56          * which in turn means that they are guaranteed to immediately
  57          * decrement their sem->read_count, so that it doesn't matter that the
  58          * writer missed them.
  59          */
  60
  61         smp_mb(); /* A matches D */
  62
  63         /*
  64          * If !readers_block the critical section starts here, matched by the
  65          * release in percpu_up_write().
  66          */
  67         if (likely(!smp_load_acquire(&sem->readers_block)))
  68                 return 1;
  69
  70         /*
  71          * Per the above comment; we still have preemption disabled and
  72          * will thus decrement on the same CPU as we incremented.
  73          */
  74         __percpu_up_read(sem);
  75
  76         if (try)
  77                 return 0;
  78
  79         /*
  80          * We either call schedule() in the wait, or we'll fall through
  81          * and reschedule on the preempt_enable() in percpu_down_read().
  82          */
  83         preempt_enable_no_resched();
  84
  85         /*
  86          * Avoid lockdep for the down/up_read() we already have them.
  87          */
  88         __down_read(&sem->rw_sem);
  89         this_cpu_inc(*sem->read_count);
  90         __up_read(&sem->rw_sem);
  91
  92         preempt_disable();
  93         return 1;
  94 }
  95 EXPORT_SYMBOL_GPL(__percpu_down_read);
  96
  97 void __percpu_up_read(struct percpu_rw_semaphore *sem)
  98 {
  99         smp_mb(); /* B matches C */
 100         /*
 101          * In other words, if they see our decrement (presumably to aggregate
 102          * zero, as that is the only time it matters) they will also see our
 103          * critical section.
 104          */
 105         __this_cpu_dec(*sem->read_count);
 106
 107         /* Prod writer to recheck readers_active */
 108         rcuwait_wake_up(&sem->writer);
 109 }
 110 EXPORT_SYMBOL_GPL(__percpu_up_read);
 111
 112 #define per_cpu_sum(var)                                                \
 113 ({                                                                      \
 114         typeof(var) __sum = 0;                                          \
 115         int cpu;                                                        \
 116         compiletime_assert_atomic_type(__sum);                          \
 117         for_each_possible_cpu(cpu)                                      \
 118                 __sum += per_cpu(var, cpu);                             \
 119         __sum;                                                          \
 120 })
 121
 122 /*
 123  * Return true if the modular sum of the sem->read_count per-CPU variable is
 124  * zero.  If this sum is zero, then it is stable due to the fact that if any
 125  * newly arriving readers increment a given counter, they will immediately
 126  * decrement that same counter.
 127  */
 128 static bool readers_active_check(struct percpu_rw_semaphore *sem)
 129 {
 130         if (per_cpu_sum(*sem->read_count) != 0)
 131                 return false;
 132
 133         /*
 134          * If we observed the decrement; ensure we see the entire critical
 135          * section.
 136          */
 137
 138         smp_mb(); /* C matches B */
 139
 140         return true;
 141 }
 142
 143 void percpu_down_write(struct percpu_rw_semaphore *sem)
 144 {
 145         /* Notify readers to take the slow path. */
 146         rcu_sync_enter(&sem->rss);
 147
 148         down_write(&sem->rw_sem);
 149
 150         /*
 151          * Notify new readers to block; up until now, and thus throughout the
 152          * longish rcu_sync_enter() above, new readers could still come in.
 153          */
 154         WRITE_ONCE(sem->readers_block, 1);
 155
 156         smp_mb(); /* D matches A */
 157
 158         /*
 159          * If they don't see our writer of readers_block, then we are
 160          * guaranteed to see their sem->read_count increment, and therefore
 161          * will wait for them.
 162          */
 163
 164         /* Wait for all now active readers to complete. */
 165         rcuwait_wait_event(&sem->writer, readers_active_check(sem));
 166 }
 167 EXPORT_SYMBOL_GPL(percpu_down_write);
 168
 169 void percpu_up_write(struct percpu_rw_semaphore *sem)
 170 {
 171         /*
 172          * Signal the writer is done, no fast path yet.
 173          *
 174          * One reason that we cannot just immediately flip to readers_fast is
 175          * that new readers might fail to see the results of this writer's
 176          * critical section.
 177          *
 178          * Therefore we force it through the slow path which guarantees an
 179          * acquire and thereby guarantees the critical section's consistency.
 180          */
 181         smp_store_release(&sem->readers_block, 0);
 182
 183         /*
 184          * Release the write lock, this will allow readers back in the game.
 185          */
 186         up_write(&sem->rw_sem);
 187
 188         /*
 189          * Once this completes (at least one RCU-sched grace period hence) the
 190          * reader fast path will be available again. Safe to use outside the
 191          * exclusive write lock because its counting.
 192          */
 193         rcu_sync_exit(&sem->rss);
 194 }
 195 EXPORT_SYMBOL_GPL(percpu_up_write);