linux-kernel - Re: [PATCH] fs: direct-io: increase bio refcount as batch

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Message-ID: <x49zj6qabtw.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 02 Apr 2015 09:39:39 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs: direct-io: increase bio refcount as batch

Ming Lei <ming.lei@...onical.com> writes:

> Hi,
>
> On Tue, Mar 31, 2015 at 10:02 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> Ming Lei <ming.lei@...onical.com> writes:
>>
>>> Each bio is always submitted to block device one by one,
>>> so it isn't necessary to increase the bio refcount by one
>>> each time with holding dio->bio_lock.
>>
>> This patch opens up a race where a completion event can come in before
>> the refcount for the dio is incremented, resulting in refcount going
>> negative.  I don't think that will actually cause problems, but it
>> certainly is ugly, and I doubt it was the intended design.
>
> Could you explain why you think it is a race and a bug? When
> dio->refcount is negative, dio_bio_end_*() only completes the
> current BIO, which is just what the function should do, isn't it?

I didn't say it was a bug.  :)  Refcounts going negative isn't something
that seems clean, though.  If you're going to push this patch through,
at least add a comment saying that this can happen by design, and is
safe.

>> Before I dig into this any further, would you care to comment on why you
>> went down this path?  Did you see spinlock contention here?  And was
>> there a resultant performance improvement for some benchmark with the
>> patch applied?
>
> It is just a minor optimization in theory, especially in case of lots of BIO
> in one dio.

It seems plausible that it would be a win.  It sure would be nice to
have some numbers, though.

Cheers,
Jeff
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