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Documentation/locking/atomic: Finish the document...

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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>
This commit is contained in:
Peter Zijlstra 2017-08-23 18:15:20 +02:00 committed by Ingo Molnar
parent e6f3faa734
commit ca110694c6
1 changed files with 42 additions and 0 deletions
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42
Documentation/atomic_t.txt
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@ -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.