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Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg()

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There seems to be a significant amount of confusion around the new
try_cmpxchg(), despite this being more like the C11
atomic_compare_exchange_*() family. Add a few words of clarification
on how cmpxchg() and try_cmpxchg() relate to one another.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/YOMgPeMOmmiK3tXO@hirez.programming.kicks-ass.net
This commit is contained in:
Peter Zijlstra 2021-07-05 17:00:24 +02:00
parent e6b4457b05
commit d1bbfd0c7c
1 changed files with 41 additions and 0 deletions
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41
Documentation/atomic_t.txt
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@ -271,3 +271,44 @@ WRITE_ONCE. Thus:
SC *y, t;
is allowed.
CMPXCHG vs TRY_CMPXCHG
----------------------
int atomic_cmpxchg(atomic_t *ptr, int old, int new);
bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new);
Both provide the same functionality, but try_cmpxchg() can lead to more
compact code. The functions relate like:
bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new)
{
int ret, old = *oldp;
ret = atomic_cmpxchg(ptr, old, new);
if (ret != old)
*oldp = ret;
return ret == old;
}
and:
int atomic_cmpxchg(atomic_t *ptr, int old, int new)
{
(void)atomic_try_cmpxchg(ptr, &old, new);
return old;
}
Usage:
old = atomic_read(&v); old = atomic_read(&v);
for (;;) { do {
new = func(old); new = func(old);
tmp = atomic_cmpxchg(&v, old, new); } while (!atomic_try_cmpxchg(&v, &old, new));
if (tmp == old)
break;
old = tmp;
}
NB. try_cmpxchg() also generates better code on some platforms (notably x86)
where the function more closely matches the hardware instruction.