linux-kernel - Re: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard

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Message-ID: <20191113102357.GA25875@willie-the-truck>
Date:   Wed, 13 Nov 2019 10:23:58 +0000
From:   Will Deacon <will@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Jens Axboe <axboe@...nel.dk>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Vincent Whitchurch <rabinv@...s.com>,
        Richard Earnshaw <Richard.Earnshaw@....com>
Subject: Re: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read
 hazard

On Tue, Nov 12, 2019 at 10:39:01AM -0800, Linus Torvalds wrote:
> On Tue, Nov 12, 2019 at 10:22 AM Catalin Marinas
> <catalin.marinas@....com> wrote:
> >
> > OK, so this includes changing test_bit() to perform a READ_ONCE.
> 
> That's not going to happen.

Ok, I'll stick my neck out here, but if test_bit() is being used to read
a bitmap that is being concurrently modified (e.g. by set_bit() which boils
down to atomic_long_or()), then why isn't READ_ONCE() required? Right now,
test_bit takes a 'const volatile unsigned long *addr' argument, so I don't
see that you'll get a change in codegen except on alpha and, with this
erratum, arm32.

Will