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authorPeter Zijlstra <peterz@infradead.org>2017-08-23 18:15:20 +0200
committerIngo Molnar <mingo@kernel.org>2017-08-25 11:06:33 +0200
commitca110694c6950dfd7bc864138c80fe39ea43da5b (patch)
treecd1924e8654b7d3bf23e33eea2c4700536039b6c
parente6f3faa734a00c606b7b06c6b9f15e5627d3245b (diff)
Documentation/locking/atomic: Finish the document...
Julia reported that the document looked unfinished, and it is. I forgot to include the example cooked up by Paul here: https://lkml.kernel.org/r/20170731174345.GL3730@linux.vnet.ibm.com and I added an explicit example showing how, while it is an ACQUIRE pattern, it really does provide an MB. Reported-by: Julia Cartwright <julia@ni.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Signed-off-by: Ingo Molnar <mingo@kernel.org>
-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.