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Subject: Re: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure
To:     Dexuan Cui <decui@microsoft.com>,
        "jejb@linux.ibm.com" <jejb@linux.ibm.com>,
        "martin.petersen@oracle.com" <martin.petersen@oracle.com>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "hch@lst.de" <hch@lst.de>, "hare@suse.de" <hare@suse.de>,
        Michael Kelley <mikelley@microsoft.com>,
        Long Li <longli@microsoft.com>,
        "ming.lei@redhat.com" <ming.lei@redhat.com>,
        Balsundar P <Balsundar.P@microchip.com>
Cc:     "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>
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From:   Bart Van Assche <bvanassche@acm.org>
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Date:   Thu, 23 Apr 2020 16:25:11 -0700
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On 2020-04-23 11:29, Dexuan Cui wrote:
> So it looks the below patch also works for me:
> 
> --- a/kernel/power/hibernate.c
> +++ b/kernel/power/hibernate.c
> @@ -898,6 +898,11 @@ static int software_resume(void)
>         error = freeze_processes();
>         if (error)
>                 goto Close_Finish;
> +
> +       error = freeze_kernel_threads();
> +       if (error)
> +               goto Close_Finish;
> +
>         error = load_image_and_restore();
>         thaw_processes();
>   Finish:
> 
> Just to be sure, I'll do more tests, but I believe the panic can be fixed
> by this according to my tests I have done so far.

If a freeze_kernel_threads() call is added in software_resume(), should
a thaw_kernel_threads() call be added too?

Anyway, please Cc me if a patch for software_resume() is submitted.

> I'm still not sure what the comment before scsi_device_quiesce() means:
>  *  ... Since special requests may also be requeued requests,
>  *      a successful return doesn't guarantee the device will be
>  *      totally quiescent.
> 
> I don't know if there can be some other I/O submitted after
> scsi_device_quiesce() returns in the case of hibernation, and I don't
> know if aac_suspend() -> scsi_host_block() should be fixed/removed,
> but as far as the panic is concerned, I'm very glad I have found a better
> fix with your help. 

The function blk_set_pm_only() increments the q->pm_only counter while
the blk_clear_pm_only() function decrements the q->pm_only counter.
If q->pm_only > 0, blk_queue_enter() only succeeds if the flag
BLK_MQ_REQ_PREEMPT is set in the second argument passed to that
function. blk_get_request() calls blk_queue_enter(). The result is that
while q->pm_only > 0 blk_get_request() only submits a request without
waiting if the BLK_MQ_REQ_PREEMPT flag is set in its second argument.
scsi_execute() sets the BLK_MQ_REQ_PREEMPT flag. In other words,
scsi_device_quiesce() blocks requests submitted by filesystems but still
allows SCSI commands submitted by the SCSI core to be executed.
"special" refers to requests with the BLK_MQ_REQ_PREEMPT flag set.

Bart.