linux-kernel - Re: [PATCH 1/2] PM / sleep: ensure deferred probe workqueue is finished in wait_for_device

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Message-ID: <2953623.dd2aFp1rBS@vostro.rjw.lan>
Date:	Fri, 09 Oct 2015 23:16:10 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 1/2] PM / sleep: ensure deferred probe workqueue is finished in wait_for_device_probe

On Friday, October 09, 2015 09:38:13 AM Grygorii Strashko wrote:
> On 10/08/2015 03:53 PM, Alan Stern wrote:
> > On Thu, 8 Oct 2015, Rafael J. Wysocki wrote:
> > 
> >>> @@ -391,6 +391,10 @@ int driver_probe_done(void)
> >>>    */
> >>>   void wait_for_device_probe(void)
> >>>   {
> >>> +	/* wait for the deferred probe workqueue to finish */
> >>> +	if (driver_deferred_probe_enable)
> >>> +		flush_workqueue(deferred_wq);
> >>> +
> >>>   	/* wait for the known devices to complete their probing */
> >>>   	wait_event(probe_waitqueue, atomic_read(&probe_count) == 0);
> >>>   	async_synchronize_full();
> >>
> >> I'm not sure if this is sufficient.
> >>
> >> Something may be added to the workqueue right after you've flushed it and
> >> then be reporobed after the wait_event() in theory.  Or am I missing anything?
> > 
> > Maybe I'm missing part of this, but I think the point is to make sure
> > that every probe which began or was queued before this function got
> > called, has finished before the function returns.
> > 
> > Thus, in the case at hand we want to defer all probes starting from
> > some point in the system sleep transition.  Grygorii sets his
> > defer_all_probes variable and then calls this function.  It waits for
> > any probes that were initiated before the function call.  Any probe
> > that was initiated after the function call (for example, the ones
> > you're concerned about between the flush_workqueue and wait_event) will
> > see that defer_all_probes is set and so will defer itself.
> 
> Yes. It will work as expected with the next patch.
> For all other case, where this API is used alone -
> it will make things more safe, but there is no way to completely block
> scheduling of new probes.

Well, in that case why don't you make it part of the second patch after all
instead of making a false impression of fixing a more general problem?

Thanks,
Rafael

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