linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Date:   Thu, 30 Aug 2018 14:46:16 +0000
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     "peterz@...radead.org" <peterz@...radead.org>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

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