Re: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

[Vineet Gupta]

On Wednesday 09 March 2016 05:10 PM, Peter Zijlstra wrote:
> On Wed, Mar 09, 2016 at 04:30:31PM +0530, Vineet Gupta wrote:
>> FWIW, could we add some background to commit log, specifically what prompted this.
>> Something like below...
> 
> Sure.. find below.
> 
>>> +++ b/include/asm-generic/bitops/lock.h
>>> @@ -29,16 +29,16 @@ do {					\
>>>   * @nr: the bit to set
>>>   * @addr: the address to start counting from
>>>   *
>>> + * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
>>> + * the bits in the word are protected by this lock some archs can use weaker
>>> + * ops to safely unlock.
>>> + *
>>> + * See for example x86's implementation.
>>>   */
>>
>> To be able to override/use-generic don't we need #ifndef ....
> 
> I did not follow through the maze, I think the few archs implementing
> this simply do not include this file at all.
> 
> I'll let the first person that cares about this worry about that :-)

Ok - that's be me :-) although I really don't see much gains in case of ARC LLSC.

For us, LD + BCLR + ST is very similar to LLOCK + BCLR + SCOND atleast in terms of
cache coherency transactions !

> 
> ---
> Subject: bitops: Do not default to __clear_bit() for __clear_bit_unlock()
> 
> __clear_bit_unlock() is a special little snowflake. While it carries the
> non-atomic '__' prefix, it is specifically documented to pair with
> test_and_set_bit() and therefore should be 'somewhat' atomic.
> 
> Therefore the generic implementation of __clear_bit_unlock() cannot use
> the fully non-atomic __clear_bit() as a default.
> 
> If an arch is able to do better; is must provide an implementation of
> __clear_bit_unlock() itself.
> 
> Specifically, this came up as a result of hackbench livelock'ing in
> slab_lock() on ARC with SMP + SLUB + !LLSC.
> 
> The issue was incorrect pairing of atomic ops.
> 
> slab_lock() -> bit_spin_lock() -> test_and_set_bit()
> slab_unlock() -> __bit_spin_unlock() -> __clear_bit()
> 
> The non serializing __clear_bit() was getting "lost"
> 
> 80543b8e:	ld_s       r2,[r13,0] <--- (A) Finds PG_locked is set
> 80543b90:	or         r3,r2,1    <--- (B) other core unlocks right here
> 80543b94:	st_s       r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)
> 
> Fixes ARC STAR 9000817404 (and probably more).
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>
> Tested-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

LGTM. Thx a bunch Peter !

-Vineet

> ---
>  include/asm-generic/bitops/lock.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
> index c30266e94806..8ef0ccbf8167 100644
> --- a/include/asm-generic/bitops/lock.h
> +++ b/include/asm-generic/bitops/lock.h
> @@ -29,16 +29,16 @@ do {					\
>   * @nr: the bit to set
>   * @addr: the address to start counting from
>   *
> - * This operation is like clear_bit_unlock, however it is not atomic.
> - * It does provide release barrier semantics so it can be used to unlock
> - * a bit lock, however it would only be used if no other CPU can modify
> - * any bits in the memory until the lock is released (a good example is
> - * if the bit lock itself protects access to the other bits in the word).
> + * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
> + * the bits in the word are protected by this lock some archs can use weaker
> + * ops to safely unlock.
> + *
> + * See for example x86's implementation.
>   */
>  #define __clear_bit_unlock(nr, addr)	\
>  do {					\
> -	smp_mb();			\
> -	__clear_bit(nr, addr);		\
> +	smp_mb__before_atomic();	\
> +	clear_bit(nr, addr);		\
>  } while (0)
>  
>  #endif /* _ASM_GENERIC_BITOPS_LOCK_H_ */
>