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

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Date:   Wed, 13 Nov 2019 08:40:41 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.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 Wed, Nov 13, 2019 at 8:36 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> test_bit() is a very unfortunate interface, in that we actually use it
> in some situations where we _really_ would want to merge reads (not
> split them, but merge them). There are several cases where we do
> constant test-bits on the same word, and don't care about ordering.
> Things like thread flags etc.

Side note: test_bit() really isn't good for locking in the first
place. The fact that the buffer heads use it for that is very
non-optimal indeed.

Particularly for testing something like "is this buffer uptodate", it
should be a "smp_load_acquire()", not a test_bit(). And READ_ONCE()
doesn't really help.

So in many ways it would be much better to make the buffer head stuff
use proper ordered accesses. But I suspect nobody is going to ever
want to go through that pain for a legacy thing, so the papering it
over with READ_ONCE() and a ugly ARM hw erratum hack is probably the
best we'll do..

                Linus