linux-kernel - Re: [uclinux-dist-devel] [PATCH/RFC] asm-generic/mutex-dec.h: add SMP support

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Date:	Mon, 6 Jun 2011 18:33:44 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-arch@...r.kernel.org, Piggin <npiggin@...nel.dk>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	Nick@...ckfin.uclinux.org, uclinux-dist-devel@...ckfin.uclinux.org
Subject: Re: [uclinux-dist-devel] [PATCH/RFC] asm-generic/mutex-dec.h: add SMP support

On Mon, Jun 6, 2011 at 17:31, Mike Frysinger wrote:
> On Mon, Jun 6, 2011 at 17:23, Andrew Morton wrote:
>> On Sun, 29 May 2011 23:19:28 -0400 Mike Frysinger wrote:
>>> To make these guys work on SMP systems, we just need to sprinkle a few
>>> barriers around.
>>>
>>> diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h
>>> index f104af7..e746c3c 100644
>>> --- a/include/asm-generic/mutex-dec.h
>>> +++ b/include/asm-generic/mutex-dec.h
>>> @@ -22,6 +22,8 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
>>>  {
>>>       if (unlikely(atomic_dec_return(count) < 0))
>>>               fail_fn(count);
>>> +     else
>>> +             smp_mb();
>>>  }
>>>
>>>  /**
>>> @@ -39,6 +41,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
>>>  {
>>>       if (unlikely(atomic_dec_return(count) < 0))
>>>               return fail_fn(count);
>>> +     smp_mb();
>>>       return 0;
>>>  }
>>>
>>> @@ -58,6 +61,7 @@ __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *))
>>>  static inline void
>>>  __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
>>>  {
>>> +     smp_mb();
>>>       if (unlikely(atomic_inc_return(count) <= 0))
>>>               fail_fn(count);
>>>  }
>>> @@ -82,8 +86,10 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
>>>  static inline int
>>>  __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
>>>  {
>>> -     if (likely(atomic_cmpxchg(count, 1, 0) == 1))
>>> +     if (likely(atomic_cmpxchg(count, 1, 0) == 1)) {
>>> +             smp_mb();
>>>               return 1;
>>> +     }
>>>       return 0;
>>>  }
>>
>> This patch basically reverts Nick's a8ddac7e53e89cb ("mutex: speed up
>> generic mutex implementations").  What's up with that?
>>
>> I could try to review this patch but I'm pathetic with barriers.  Help.
>
> thanks for that tip.  i think we can chalk this patch up to the
> origins of the Blackfin SMP port ... it branched this code before
> Nick's patch, and never incorporated common changes back.  so i'll
> just drop it once i boot up a system to double check and convert the
> Blackfin code over to the asm-generic version completely to avoid
> future issues.

seems to be ok (and our core atomic's do include barriers themselves),
so let's just drop this patch on the floor and forget about it.
thanks all!
-mike
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