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

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Date:   Tue, 18 Feb 2020 09:12:59 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Merlijn Wajer <merlijn@...hive.org>
Cc:     merlijn@...zup.org, linux-scsi@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "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 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?