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-rw-r--r--Documentation/atomic_t.txt42
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.