Commit-ID: ca110694c6950dfd7bc864138c80fe39ea43da5b
Gitweb: http://git.kernel.org/tip/ca110694c6950dfd7bc864138c80fe39ea43da5b
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 23 Aug 2017 18:15:20 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 25 Aug 2017 11:06:33 +0200
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@xxxxxxxxxxxxxxxxxx
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@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
Documentation/atomic_t.txt | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index eee1271..913396a 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.
--
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