Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Dmitry Vyukov]

On Mon, Mar 27, 2017 at 2:16 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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)


Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>