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

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Date:   Tue, 18 Feb 2020 09:20:28 -0800
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Merlijn Wajer <merlijn@...hive.org>
Cc:     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, 2020-02-18 at 09:12 -0800, Christoph Hellwig wrote:
> On Tue, Feb 18, 2020 at 03:39:17PM +0100, Merlijn Wajer wrote:
> > When replacing the Big Kernel Lock in commit
> > 2a48fc0ab24241755dc93bfd4f01d68efab47f5a ("block: autoconvert
> > trivial BKL users to private mutex"), the lock was replaced with a
> > sr-wide lock.
> > 
> > This causes very poor performance when using multiple sr devices,
> > as the sr driver was not able to execute more than one command to
> > one drive at any given time, even when there were many CD drives
> > available.
> > 
> > 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.

I think the ioctls don't need the mutex (not looked deeply enough) and
certainly the probe only requires it for the idr allocation which has
its own lock, so I don't believe the mutex additions are needed there.

James