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| -rw-r--r-- | Documentation/atomic_t.txt | 42 |
1 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt index eee127115277..913396ac5824 100644 --- a/Documentation/atomic_t.txt +++ b/Documentation/atomic_t.txt @@ -197,4 +197,46 @@ Further, while something like: is a 'typical' RELEASE pattern, the barrier is strictly stronger than a RELEASE. Similarly for something like: + atomic_inc(&X); + smp_mb__after_atomic(); + +is an ACQUIRE pattern (though very much not typical), but again the barrier is +strictly stronger than ACQUIRE. As illustrated: + + C strong-acquire + + { + } + + P1(int *x, atomic_t *y) + { + r0 = READ_ONCE(*x); + smp_rmb(); + r1 = atomic_read(y); + } + + P2(int *x, atomic_t *y) + { + atomic_inc(y); + smp_mb__after_atomic(); + WRITE_ONCE(*x, 1); + } + + exists + (r0=1 /\ r1=0) + +This should not happen; but a hypothetical atomic_inc_acquire() -- +(void)atomic_fetch_inc_acquire() for instance -- would allow the outcome, +since then: + + P1 P2 + + t = LL.acq *y (0) + t++; + *x = 1; + r0 = *x (1) + RMB + r1 = *y (0) + SC *y, t; +is allowed. |