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Date:   Wed, 12 Jun 2019 00:01:10 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     John Dorminy <jdorminy@redhat.com>
Cc:     Mike Snitzer <snitzer@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        NeilBrown <neilb@suse.com>, linux-block@vger.kernel.org,
        device-mapper development <dm-devel@redhat.com>,
        Milan Broz <gmazyland@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: block: be more careful about status in __bio_chain_endio
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On Tue, Jun 11, 2019 at 10:56:42PM -0400, John Dorminy wrote:
> I believe the second of these might, but is not guaranteed to,
> preserve the first error observed in a child; I believe if you want to
> definitely save the first error you need an atomic.

Is there any reason not to simply use a cmpxchg?  Yes, it is a
relatively expensive operation, but once we are chaining bios we are out
of the super hot path anyway.  We do something similar in xfs and iomap
already.