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Date:   Thu, 30 Aug 2018 11:44:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.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 Wed, Aug 29, 2018 at 09:16:43PM +0000, Vineet Gupta wrote:
> On 08/29/2018 11:33 AM, Eugeniy Paltsev wrote:
> > Hi Guys,
> > Since v4.19-rc1 we are getting a serious regression on platforms with ARC architecture.
> > The kernel have become unstable and spontaneously crashes on LTP tests execution / IO tests or
> > even on boot.
> >
> > I don't know exactly what breaks but bisect clearly assign the blame to this commit:
> > 84c6591103db ("locking/atomics, asm-generic/bitops/lock.h: Rewrite using atomic_fetch_*()")
> > https://github.com/torvalds/linux/commit/84c6591103dbeaf393a092a3fc7b09510825f6b9
> >
> > Reverting the commit solves this problem.
> >
> > I tested v4.19-rc1 on ARM (wandboard, i.mx6, 32bit, quard core, ARMv7) which uses same
> > generic bitops implementation and it works fine.
> >
> > Do you have any ideas what went wrong?
> 
> Back in 2016, Peter had fixed this file due to a problem I reported on ARC. See
> commit f75d48644c56a ("bitops: Do not default to __clear_bit() for
> __clear_bit_unlock()")
> That made __clear_bit_unlock() use the atomic clear_bit() vs. non-atomic
> __clear_bit(), effectively making clear_bit_unlock() and __clear_bit_unlock() same.
> 
> This patch undoes that which could explain the issues you see. @Peter, @Will ?

Right, so the thinking is that on platforms that suffer that issue,
atomic_set*() should DTRT. And if you look at your spinlock based atomic
implementation, you'll note that atomic_set() does indeed do the right
thing.

arch/arc/include/asm/atomic.h:108




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