linux-kernel - Re: Cache coherency bug: stale reads on /dev/sda1

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Date:   Sun, 27 Aug 2023 03:06:35 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Joshua Hudson <joshudson@...il.com>
Cc:     Bagas Sanjaya <bagasdotme@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux IDE and libata <linux-ide@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Damien Le Moal <dlemoal@...nel.org>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: Cache coherency bug: stale reads on /dev/sda1

On Sat, Aug 26, 2023 at 06:25:58PM -0700, Joshua Hudson wrote:
> I am presuming you mean "Are you sure this not a hardware issue?"
> 
> I am sure it is not, for two reasons.
> 
> 1) If it were a hardware issue I would still expect the two device
> nodes (whole disk and partition) to report the *same* data.
> 
> 2) I have since developed a workaround involving BLKFLSDEV. The
> workaround is really ugly.

Whole disk and all partitions have page caches of their own.
There is no coherency between those; write through one, then
read through another and you are not guaranteed that read won't
see the stale cached data.

Doctor, it hurts when I do it...

Incidentally, read from device/write to the place you've read via
file/read from device again also has no coherency warranties, for
exact same reason.

IOW, not a bug.