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

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Message-ID: <20191120191839.GF4799@willie-the-truck>
Date:   Wed, 20 Nov 2019 19:18:40 +0000
From:   Will Deacon <will@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     linux@...linux.org.uk, torvalds@...ux-foundation.org,
        catalin.marinas@....com, vincent.whitchurch@...s.com,
        axboe@...nel.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, rabinv@...s.com,
        Richard.Earnshaw@....com
Subject: Re: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read
 hazard

On Thu, Nov 14, 2019 at 09:28:48PM +0800, Herbert Xu wrote:
> Will Deacon <will@...nel.org> wrote:
> >
> > which is what can happen due to this erratum. It's generally good practice
> > to use READ_ONCE() when reading something which can be updated concurrently
> > because:
> > 
> >        * It ensures that the value is (re-)loaded from memory
> > 
> >        * It prevents the compiler from performing harmful optimisations,
> >          such as merging or tearing (although in this case I suspect
> >          these are ok because we're dealing with a single bit)
> > 
> >        * On Alpha, it gives you a barrier so that dependency ordering
> >          can be relied upon from the load
> 
> The Alpha barrier matters for pointers, how could it make a
> difference for individual bits?

I guess you could use the result of test_bit to index into an array or
something?

Will