linux-kernel - Re: [PATCH 2/2] PM / sleep: prohibit devices probing during suspend/hibernation

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Date:	Thu, 8 Oct 2015 15:20:08 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Grygorii Strashko <grygorii.strashko@...com>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 2/2] PM / sleep: prohibit devices probing during
 suspend/hibernation

On Thu, 8 Oct 2015, Grygorii Strashko wrote:

> >>   /**
> >> + * device_defer_all_probes() - Enable/disable probing of devices
> >> + * @enable:  Enable/disable probing of devices
> >> + *
> >> + * if @enable = true
> >> + *	It will disable probing of devices and defer their probes.
> >> + * otherwise
> >> + *	It will restore normal behavior and trigger re-probing of deferred
> >> + *	devices.
> >> + */
> >> +void device_defer_all_probes(bool enable)
> >> +{
> >> +	defer_all_probes = enable;
> >> +	if (enable)
> >> +		/* sync with probes to avoid any races. */
> >> +		wait_for_device_probe();
> 
> ^ pls, pay attention on above code line
> 
> >> +	else
> >> +		driver_deferred_probe_trigger();
> >> +}
> > 
> > Some people might prefer to see two separate functions, an enable
> > routine and a disable routine.  I don't much care.
> 
> May be. Should I change it?

It would then be more in line with functions like 
pm_runtime_set_{active|suspended} or pm_runtime_[dont_]use_autosuspend.

> >> @@ -277,9 +304,15 @@ static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
> >>   
> >>   static int really_probe(struct device *dev, struct device_driver *drv)
> >>   {
> >> -	int ret = 0;
> >> +	int ret = -EPROBE_DEFER;
> >>   	int local_trigger_count = atomic_read(&deferred_trigger_count);
> >>   
> >> +	if (defer_all_probes) {
> >> +		dev_dbg(dev, "Driver %s force probe deferral\n", drv->name);
> >> +		driver_deferred_probe_add(dev);
> >> +		return ret;
> >> +	}
> > 
> > In theory there's a race here.  If one CPU sets defer_all_probes, the
> > new value might not be perceived by another CPU until a little while
> > later.  Is there an easy way to insure that this race won't cause any
> > problems?
> 
> Yes. this question was raised by Rafael also [1].

I see.  Can you add a comment explaining all of this?

Alan Stern

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