linux-kernel - Re: [PATCH v2 2/2] scsi: sd: Rework asynchronous resume support

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Date:   Wed, 20 Jul 2022 11:04:33 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        scsi <linux-scsi@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>, Hannes Reinecke <hare@...e.de>,
        John Garry <john.garry@...wei.com>, ericspero@...oud.com,
        jason600.groome@...il.com,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] scsi: sd: Rework asynchronous resume support

On 7/20/22 10:44, Geert Uytterhoeven wrote:
> On Wed, Jul 20, 2022 at 6:51 PM Bart Van Assche <bvanassche@....org> wrote:
>> I'm not familiar with the SATA code but from a quick look it seems like
>> the above code is only triggered from inside the ATA error handler
>> (ata_do_eh() -> ata_eh_recover() -> ata_eh_revalidate_and_attach() ->
>> schedule_work(&(ap->scsi_rescan_task) -> ata_scsi_dev_rescan()). It
>> doesn't seem normal to me that the ATA error handler gets invoked during
>> a resume. How about testing the following two code changes?
> 
> Thanks for your suggestions!
> 
>> * In sd_start_stop_device(), change "return sd_submit_start(sdkp, cmd,
>> sizeof(cmd))" into "sd_submit_start(sdkp, cmd, sizeof(cmd))" and below
>> that call add "flush_work(&sdkp->start_done_work)". This makes
>> sd_start_stop_device() again synchronous. This will learn us whether the
>> behavior change is caused by submitting the START command from another
>> context or by not waiting until the START command has finished.
> 
> Unfortunately this doesn't have any impact.
> 
>> * Back out the above change, change "return sd_submit_start(sdkp, cmd,
>> sizeof(cmd))" again into "sd_submit_start(sdkp, cmd, sizeof(cmd))" and
>> below that statement add a call to
>> scsi_run_queue(sdkp->device->request_queue). If this change helps it
> 
> (that's the static scsi_run_queue() in drivers/scsi/scsi_lib.c?)
> 
>> means that the scsi_run_queue() call is necessary to prevent reordering
>> of the START command with other SCSI commands.
> 
> Unfortunately this doesn't have any impact either.

That's surprising. Is there anything unusual about the test setup that I 
should know, e.g. very small number of CPU cores or a very small queue 
depth of the SATA device? How about adding pr_info() statements at the 
start and end of the following functions and also before the return 
statements in these functions to determine where execution of the START 
command hangs?
* sd_start_done().
* sd_start_done_work().

Thanks,

Bart.