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From: Bart Van Assche <bvanassche@acm.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: linux-scsi@vger.kernel.org,
	Martin Kepplinger <martin.kepplinger@puri.sm>,
	Can Guo <cang@codeaurora.org>
Subject: Re: [PATCH RFC 6/6] block, scsi, ide: Only submit power management requests in state RPM_SUSPENDED
Date: Mon, 31 Aug 2020 22:00:31 -0700	[thread overview]
Message-ID: <e84a8098-0c5f-93fb-9055-292104cb2483@acm.org> (raw)
In-Reply-To: <20200831182526.GA558270@rowland.harvard.edu>

On 2020-08-31 11:25, Alan Stern wrote:
> Let me clarify this description.
> 
> Firstly, the second-to-last sentence is ambiguous.  The word "only" is
> all too easy to misuse through carelessness.  In this case you meant
> to say "by accepting only power management requests while suspended",
> but what you actually wrote was equivalent to "by accepting power
> management requests only while suspended".  And as it happens, both
> meanings are incorrect because we don't want to accept _any_ requests
> while the device is suspended -- not even power-management requests.
> The description should have said "by postponing all non-power-management 
> requests while the device is suspending, suspended, or resuming."
> 
> Secondly, the scenario described above is not a deadlock; it is a race 
> leading to a command failure.  Namely, the thread setting q->rpm_status 
> to RPM_SUSPENDED races with the thread testing q->rpm_status.
> 
> Thirdly, this race is _not_ the problem that Martin encountered.  His 
> problem was a simple bug (failure to postpone a request), not a race or 
> a deadlock.
> 
> Fourthly, the race illustrated above is, for now, theoretical.  It 
> cannot occur with the existing code base (mostly because the existing 
> code is buggy).  The advantage of your series over the patch I submitted 
> on Aug. 23 ("block: Fix bug in runtime-resume handling") is that it 
> fixes Martin's problem without introducing this new race.

Hi Alan,

Thanks for having taken a look at this patch. Do you perhaps plan to
review the other patches in this series too?

Thanks,

Bart.

  reply	other threads:[~2020-09-01  5:00 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-31  2:53 [PATCH RFC 0/6] Fix a deadlock in the SCSI power management code Bart Van Assche
2020-08-31  2:53 ` [PATCH RFC 1/6] ide: Do not set the RQF_PREEMPT flag for sense requests Bart Van Assche
2020-08-31  2:53 ` [PATCH RFC 2/6] scsi: Remove an incorrect comment Bart Van Assche
2020-08-31  2:53 ` [PATCH RFC 3/6] scsi: Pass a request queue pointer to __scsi_execute() Bart Van Assche
2020-08-31  2:53 ` [PATCH RFC 4/6] scsi_transport_spi: Make spi_execute() accept a request queue pointer Bart Van Assche
2020-08-31  2:53 ` [PATCH RFC 5/6] scsi_transport_spi: Freeze request queues instead of quiescing Bart Van Assche
2020-09-03  1:39   ` Alan Stern
2020-08-31  2:53 ` [PATCH RFC 6/6] block, scsi, ide: Only submit power management requests in state RPM_SUSPENDED Bart Van Assche
2020-08-31 18:25   ` Alan Stern
2020-09-01  5:00     ` Bart Van Assche [this message]
2020-09-01 14:50       ` Alan Stern
2020-08-31  9:09 ` [PATCH RFC 0/6] Fix a deadlock in the SCSI power management code Martin Kepplinger
2020-09-01  3:55   ` Bart Van Assche

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