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

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Date:   Mon, 24 Feb 2020 22:20:47 +0100
From:   "Merlijn B.W. Wajer" <merlijn@...hive.org>
To:     "Merlijn B.W. Wajer" <merlijn@...zup.org>,
        Christoph Hellwig <hch@...radead.org>,
        James Bottomley <jejb@...ux.ibm.com>
Cc:     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

Hi Martin,

Just wanted to check if you planned to apply this v2 (you tried to apply
v1 but it didn't compile, so I rebased it onto 5.7/scsi-queue as you
requested). Please let me know if there's anything you'd like to see
changed.

Regards,
Merlijn

On 18/02/2020 20:21, Merlijn B.W. Wajer wrote:
> Hi,
> 
> On 18/02/2020 18:31, Christoph Hellwig wrote:
>> 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.
> 
> The are two reasons I decided to make minor changes to fix the
> performance regression.
> 
> First, being able to send the patch to the various stable branches once
> merged. For people working with many CD drives attached to one station,
> this is a pretty big deal, so I tried to keep the patch simple. It fixes
> the regression introduced in another commit.
> 
> Secondly, I don't have the hardware to test sophisticated or old setups,
> like some of the issues linked from my patch. I have SATA CD drives with
> USB->SATA bridges, no IDE, no PATA, etc. So the testing I can do is
> relatively limited.
> 
> Perhaps I or someone else can work on removing the usage of the locks,
> but as it stands I think this addresses the performance issue present in
> the current kernel, and removing locks and the associated testing
> required with that is something I am not entirely comfortable doing.
> 
> Cheers,
> Merlijn
>