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locking/atomic: Fix atomic_try_cmpxchg() semantics 

Dmitry noted that the new atomic_try_cmpxchg() primitive is broken when
the old pointer doesn't point to the local stack.

He writes:

  "Consider a classical lock-free stack push:

    node->next = atomic_read(&head);
    do {
    } while (!atomic_try_cmpxchg(&head, &node->next, node));

  This code is broken with the current implementation, the problem is
  with unconditional update of *__po.

  In case of success it writes the same value back into *__po, but in
  case of cmpxchg success we might have lose ownership of some memory
  locations and potentially over what __po has pointed to. The same
  holds for the re-read of *__po. "

He also points out that this makes it surprisingly different from the
similar C/C++ atomic operation.

After investigating the code-gen differences caused by this patch; and
a number of alternatives (Linus dislikes this interface lots), we
arrived at these results (size x86_64-defconfig/vmlinux):

  GCC-6.3.0:

  10735757        cmpxchg
  10726413        try_cmpxchg
  10730509        try_cmpxchg + patch
  10730445        try_cmpxchg-linus

  GCC-7 (20170327):

  10709514        cmpxchg
  10704266        try_cmpxchg
  10704266        try_cmpxchg + patch
  10704394        try_cmpxchg-linus

From this we see that the patch has the advantage of better code-gen
on GCC-7 and keeps the interface roughly consistent with the C
language variant.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
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: linux-kernel@vger.kernel.org
Fixes: a9ebf306f5 ("locking/atomic: Introduce atomic_try_cmpxchg()")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2017-03-27 13:54:38 +02:00 committed by Ingo Molnar
parent 8ce371f984
commit 44fe84459f
2 changed files with 13 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
arch/x86/include/asm/cmpxchg.h
View File

@ -212,8 +212,9 @@ extern void __add_wrong_size(void)
default: \
__cmpxchg_wrong_size(); \
} \
*_old = __old; \
success; \
if (unlikely(!success)) \
*_old = __old; \
likely(success); \
})
#define __try_cmpxchg(ptr, pold, new, size) \

16
include/linux/atomic.h
View File

@ -428,9 +428,11 @@
#define __atomic_try_cmpxchg(type, _p, _po, _n) \
({ \
typeof(_po) __po = (_po); \
typeof(*(_po)) __o = *__po; \
*__po = atomic_cmpxchg##type((_p), __o, (_n)); \
(*__po == __o); \
typeof(*(_po)) __r, __o = *__po; \
__r = atomic_cmpxchg##type((_p), __o, (_n)); \
if (unlikely(__r != __o)) \
*__po = __r; \
likely(__r == __o); \
})
#define atomic_try_cmpxchg(_p, _po, _n) __atomic_try_cmpxchg(, _p, _po, _n)
@ -1022,9 +1024,11 @@ static inline int atomic_dec_if_positive(atomic_t *v)
#define __atomic64_try_cmpxchg(type, _p, _po, _n) \
({ \
typeof(_po) __po = (_po); \
typeof(*(_po)) __o = *__po; \
*__po = atomic64_cmpxchg##type((_p), __o, (_n)); \
(*__po == __o); \
typeof(*(_po)) __r, __o = *__po; \
__r = atomic64_cmpxchg##type((_p), __o, (_n)); \
if (unlikely(__r != __o)) \
*__po = __r; \
likely(__r == __o); \
})
#define atomic64_try_cmpxchg(_p, _po, _n) __atomic64_try_cmpxchg(, _p, _po, _n)