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

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Message-ID: <20200218172347.GA3020@infradead.org>
Date:   Tue, 18 Feb 2020 09:23:47 -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 09:20:28AM -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?