Subject: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_" causes kernel crash

From: [email-masked] (Eugeniy Paltsev)
Date: Thu, 30 Aug 2018 14:46:16 +0000

On Thu, 2018-08-30 at 16:17 +0200, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 11:53:17AM +0000, Eugeniy Paltsev wrote:
> > I can see crashes with LLSC enabled in both SMP running on 4 cores
> > and SMP running on 1 core.
> 
> So you're running on LL/SC enabled hardware; that would make Will's
> patch irrelevant (although still a good idea for the hardware that does
> care about that spinlocked atomic crud).
> 
> Does something like the below cure things? That would confirm the
> suggestion that the change to __clear_bit_unlock() is the curprit.

I tested it - this doesn't change anything, the problem still reproduces.

I'll test it with last Will fix.

> If that doesn't cure things, then we've been looking in entirely the
> wrong place.
> 
> ---
> diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
> index 3ae021368f48..79c6978152f8 100644
> --- a/include/asm-generic/bitops/lock.h
> +++ b/include/asm-generic/bitops/lock.h
> @@ -57,12 +57,7 @@ static inline void clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
>  static inline void __clear_bit_unlock(unsigned int nr,
>  				      volatile unsigned long *p)
>  {
> -	unsigned long old;
> -
> -	p += BIT_WORD(nr);
> -	old = READ_ONCE(*p);
> -	old &= ~BIT_MASK(nr);
> -	atomic_long_set_release((atomic_long_t *)p, old);
> +	clear_bit_unlock(nr, p);
>  }
>  
>  /**
-- 
 Eugeniy Paltsev