summaryrefslogtreecommitdiff
path: root/include/linux/atomic.h
diff options
context:
space:
mode:
authorPeter Zijlstra <peterz@infradead.org>2017-03-27 14:54:38 +0300
committerIngo Molnar <mingo@kernel.org>2017-03-30 10:35:54 +0300
commit44fe84459faf1a7781595b7c64cd36daf2f2827d (patch)
tree1fa467e66d19c23bfea059c11ebb4fee9386de41 /include/linux/atomic.h
parent8ce371f9846ef1e8b3cc8f6865766cb5c1f17e40 (diff)
downloadlinux-44fe84459faf1a7781595b7c64cd36daf2f2827d.tar.xz
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: a9ebf306f52c ("locking/atomic: Introduce atomic_try_cmpxchg()") Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'include/linux/atomic.h')
-rw-r--r--include/linux/atomic.h16
1 files changed, 10 insertions, 6 deletions
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index aae5953817d6..c56be7410130 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -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)