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

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Message-ID: <CAK8P3a2eBrAoksG11LE0CMSKg4ZW1GYTj4N2SmJ-CYQfP1-c8A@mail.gmail.com>
Date:   Fri, 14 Feb 2020 10:03:18 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Merlijn Wajer <merlijn@...zup.org>
Cc:     linux-scsi <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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] scsi: sr: get rid of sr global mutex

On Wed, Feb 12, 2020 at 5:45 PM Merlijn Wajer <merlijn@...zup.org> 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.
>
> Someone tried this patch at the time, but it never made it
> upstream, due to possible concerns with race conditions, but it's not
> clear the patch actually caused those:
>
> https://www.spinics.net/lists/linux-scsi/msg63706.html
> https://www.spinics.net/lists/linux-scsi/msg63750.html
>
> Also see
>
> http://lists.xiph.org/pipermail/paranoia/2019-December/001647.html
>
> Signed-off-by: Merlijn Wajer <merlijn@...zup.org>

This all looks reasonable to me. The conversion from BKL to a per-driver
mutex was done in a mostly automated way, and I did not attempt to make
it more fine-grained then.

I don't see any global state accessed in the open/close/ioctl functions,
so this is probably completely safe. It may even be possible to avoid
that mutex completely, but that is harder to prove.

Acked-by: Arnd Bergmann <arnd@...db.de>