Messages in this thread Patch in this message | | | From | Dmitry Vyukov <> | Date | Fri, 24 Mar 2017 13:44:00 +0100 | Subject | re: locking/atomic: Introduce atomic_try_cmpxchg() |
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Hi,
I've come across:
commit a9ebf306f52c756c4f9e50ee9a60cd6389d71344 Author: Peter Zijlstra Date: Wed Feb 1 16:39:38 2017 +0100 locking/atomic: Introduce atomic_try_cmpxchg()
The primitive has subtle difference with all other implementation that I know of, and can lead to very subtle bugs. Some time ago I've spent several days debugging a memory corruption caused by similar implementation. 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 here:
#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); \ })
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. The problem is very easy to overlook in user code (not saying about the case when developer is already familiar with different semantics of similar functions (e.g. C/C++ atomic operations, or gcc/clang atomic builtins)). For this reason cmpxchg implementations usually update comparand only in case of failure. So I would suggest to change it to a safer and less surprising alternative:
diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h index fb961db51a2a..81fb985f51f4 100644 --- a/arch/x86/include/asm/cmpxchg.h +++ b/arch/x86/include/asm/cmpxchg.h @@ -212,7 +212,8 @@ extern void __add_wrong_size(void) default: \ __cmpxchg_wrong_size(); \ } \ - *_old = __old; \ + if (!success) \ + *_old = __old; \ success; \ })
diff --git a/include/linux/atomic.h b/include/linux/atomic.h index aae5953817d6..f8098157f7c8 100644 --- a/include/linux/atomic.h +++ b/include/linux/atomic.h @@ -1023,8 +1023,11 @@ static inline int atomic_dec_if_positive(atomic_t *v) ({ \ typeof(_po) __po = (_po); \ typeof(*(_po)) __o = *__po; \ - *__po = atomic64_cmpxchg##type((_p), __o, (_n)); \ - (*__po == __o); \ + typeof(*(_po)) __v = atomic64_cmpxchg##type((_p), __o, (_n)); \ + if (__v == __o) \ + return true; \ + *__po = __v; \ + return false; \ })
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