diff options
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/atomic.h | 3 | ||||
| -rw-r--r-- | include/linux/spinlock.h | 36 |
2 files changed, 39 insertions, 0 deletions
diff --git a/include/linux/atomic.h b/include/linux/atomic.h index c56be7410130..40d6bfec0e0d 100644 --- a/include/linux/atomic.h +++ b/include/linux/atomic.h @@ -38,6 +38,9 @@ * Besides, if an arch has a special barrier for acquire/release, it could * implement its own __atomic_op_* and use the same framework for building * variants + * + * If an architecture overrides __atomic_op_acquire() it will probably want + * to define smp_mb__after_spinlock(). */ #ifndef __atomic_op_acquire #define __atomic_op_acquire(op, args...) \ diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index d9510e8522d4..840281095933 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -130,6 +130,42 @@ do { \ #define smp_mb__before_spinlock() smp_wmb() #endif +/* + * This barrier must provide two things: + * + * - it must guarantee a STORE before the spin_lock() is ordered against a + * LOAD after it, see the comments at its two usage sites. + * + * - it must ensure the critical section is RCsc. + * + * The latter is important for cases where we observe values written by other + * CPUs in spin-loops, without barriers, while being subject to scheduling. + * + * CPU0 CPU1 CPU2 + * + * for (;;) { + * if (READ_ONCE(X)) + * break; + * } + * X=1 + * <sched-out> + * <sched-in> + * r = X; + * + * without transitivity it could be that CPU1 observes X!=0 breaks the loop, + * we get migrated and CPU2 sees X==0. + * + * Since most load-store architectures implement ACQUIRE with an smp_mb() after + * the LL/SC loop, they need no further barriers. Similarly all our TSO + * architectures imply an smp_mb() for each atomic instruction and equally don't + * need more. + * + * Architectures that can implement ACQUIRE better need to take care. + */ +#ifndef smp_mb__after_spinlock +#define smp_mb__after_spinlock() do { } while (0) +#endif + /** * raw_spin_unlock_wait - wait until the spinlock gets unlocked * @lock: the spinlock in question. |