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

[Mike Frysinger <vapier@gentoo.org>]

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.
-mike