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

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Message-ID: <20200218173158.GA18386@infradead.org>
Date:   Tue, 18 Feb 2020 09:31:58 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Merlijn Wajer <merlijn@...hive.org>, merlijn@...zup.org,
        linux-scsi@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] scsi: sr: get rid of sr global mutex

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.