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

[Vineet Gupta]

On Wednesday 09 March 2016 03:43 PM, Peter Zijlstra wrote:
>>> If you take the lock in __bit_spin_unlock
>>> then the race cannot happen.
>>
>> Of course it won't but that means we penalize all non atomic callers of the API
>> with a superfluous spinlock which is not require din first place given the
>> definition of API.
> 
> Quite. _However_, your arch is still broken, but not by your fault. Its
> the generic-asm code that is wrong.
> 
> The thing is that __bit_spin_unlock() uses __clear_bit_unlock(), which
> defaults to __clear_bit(). Which is wrong.
> 
> ---
> 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.
> 
> Reported-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>

This needs to be CCed stable as it fixes a real bug for ARC.

> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Tested-by: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>

FWIW, could we add some background to commit log, specifically what prompted this.
Something like below...

---->8------
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
---->8------

> ---
>  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.
>   */

To be able to override/use-generic don't we need #ifndef ....

>  #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_ */
>