linux-kernel - Re: [PATCH v2] scsi: sr: get rid of sr global mutex

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Date:   Tue, 18 Feb 2020 20:46:06 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     "Merlijn B.W. Wajer" <merlijn@...hive.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        Merlijn Wajer <merlijn@...zup.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] scsi: sr: get rid of sr global mutex

On Tue, Feb 18, 2020 at 8:20 PM Merlijn B.W. Wajer <merlijn@...hive.org> wrote:
> On 18/02/2020 18:31, Christoph Hellwig wrote:
> > On Tue, Feb 18, 2020 at 09:28:34AM -0800, James Bottomley wrote:
> >> On Tue, 2020-02-18 at 09:23 -0800, Christoph Hellwig wrote:
> >>> On Tue, Feb 18, 2020 at 09:20:28AM -0800, James Bottomley wrote:
> >>>>>> Replace the global mutex with per-sr-device mutex.
> >>>>>
> >>>>> Do we actually need the lock at all?  What is protected by it?
> >>>>
> >>>> We do at least for cdrom_open.  It modifies the cdi structure with
> >>>> no other protection and concurrent modification would at least
> >>>> screw up the use counter which is not atomic.  Same reasoning for
> >>>> cdrom_release.
> >>>
> >>> Wouldn't the right fix to add locking to cdrom_open/release instead
> >>> of having an undocumented requirement for the callers?
> >>
> >> Yes ... but that's somewhat of a bigger patch because you now have to
> >> reason about the callbacks within cdrom.  There's also the question of
> >> whether you can assume ops->generic_packet() has its own concurrency
> >> protections ... it's certainly true for SCSI, but is it for anything
> >> else?  Although I suppose you can just not care and run the internal
> >> lock over it anyway.
> >
> > We have 4 instances of struct cdrom_device_ops in the kernel, one of
> > which has a no-op generic_packet.  So I don't think this should be a
> > huge project.
>
> The are two reasons I decided to make minor changes to fix the
> performance regression.
>
> First, being able to send the patch to the various stable branches once
> merged. For people working with many CD drives attached to one station,
> this is a pretty big deal, so I tried to keep the patch simple. It fixes
> the regression introduced in another commit.
>
> Secondly, I don't have the hardware to test sophisticated or old setups,
> like some of the issues linked from my patch. I have SATA CD drives with
> USB->SATA bridges, no IDE, no PATA, etc. So the testing I can do is
> relatively limited.
>
> Perhaps I or someone else can work on removing the usage of the locks,
> but as it stands I think this addresses the performance issue present in
> the current kernel, and removing locks and the associated testing
> required with that is something I am not entirely comfortable doing.

I think this is entirely reasonable. There is a good chance that the
per-device lock is not needed, but there is an even higher chance
that there is never any contention, because the normal use case
is for a CDROM driver is to only have one process working on it at
a time using ioctl.

        Arnd