Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Peter Zijlstra]

On Fri, Mar 24, 2017 at 01:44:00PM +0100, Dmitry Vyukov wrote:
> 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. 

OK, so how about this?

---
Subject: atomic: Fix try_cmpxchg semantics
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Mon Mar 27 13:54:38 CEST 2017

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

Fixes: a9ebf306f52c ("locking/atomic: Introduce atomic_try_cmpxchg()")
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/cmpxchg.h |    5 +++--
 include/linux/atomic.h         |   16 ++++++++++------
 2 files changed, 13 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -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)				\
--- 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
 #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)