Date:	Fri, 01 Jul 2011 09:22:36 -0700
From:	Kevin Hilman <khilman@...com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Greg KH <greg@...ah.com>, LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 3/3] PM: Limit race conditions between runtime PM and system sleep

"Rafael J. Wysocki" <rjw@...k.pl> writes:

> From: Rafael J. Wysocki <rjw@...k.pl>
>
> One of the roles of the PM core is to prevent different PM callbacks
> executed for the same device object from racing with each other.
> Unfortunately, after commit e8665002477f0278f84f898145b1f141ba26ee26
> (PM: Allow pm_runtime_suspend() to succeed during system suspend)
> runtime PM callbacks may be executed concurrently with system
> suspend/resume callbacks for the same device.
>
> The main reason for commit e8665002477f0278f84f898145b1f141ba26ee26
> was that some subsystems and device drivers wanted to use runtime PM
> helpers, pm_runtime_suspend() and pm_runtime_put_sync() in
> particular, for carrying out the suspend of devices in their
> .suspend() callbacks.  However, as it's been determined recently,
> there are multiple reasons not to do so, inlcuding:
>
>  * The caller really doesn't control the runtime PM usage counters,
>    because user space can access them through sysfs and effectively
>    block runtime PM.  That means using pm_runtime_suspend() or
>    pm_runtime_get_sync() to suspend devices during system suspend
>    may or may not work.
>
>  * If a driver calls pm_runtime_suspend() from its .suspend()
>    callback, it causes the subsystem's .runtime_suspend() callback to
>    be executed, which leads to the call sequence:
>
>    subsys->suspend(dev)
>       driver->suspend(dev)
>          pm_runtime_suspend(dev)
>             subsys->runtime_suspend(dev)
>
>    recursive from the subsystem's point of view.  For some subsystems
>    that may actually work (e.g. the platform bus type), but for some
>    it will fail in a rather spectacular fashion (e.g. PCI).  In each
>    case it means a layering violation.
>
>  * Both the subsystem and the driver can provide .suspend_noirq()
>    callbacks for system suspend that can do whatever the
>    .runtime_suspend() callbacks do just fine, so it really isn't
>    necessary to call pm_runtime_suspend() during system suspend.
>
>  * The runtime PM's handling of wakeup devices is usually different
>    from the system suspend's one, so .runtime_suspend() may simply be
>    inappropriate for system suspend.
>
>  * System suspend is supposed to work even if CONFIG_PM_RUNTIME is
>    unset.
>
>  * The runtime PM workqueue is frozen before system suspend, so if
>    whatever the driver is going to do during system suspend depends
>    on it, that simply won't work.

Thanks for the thorough description of all these reasons.

> Still, there is a good reason to allow pm_runtime_resume() to
> succeed during system suspend and resume (for instance, some
> subsystems and device drivers may legitimately use it to ensure that
> their devices are in full-power states before suspending them).
> Moreover, there is no reason to prevent runtime PM callbacks from
> being executed in parallel with the system suspend/resume .prepare()
> and .complete() callbacks and the code removed by commit
> e8665002477f0278f84f898145b1f141ba26ee26 went too far in this
> respect.  On the other hand, runtime PM callbacks, including
> .runtime_resume(), must not be executed during system suspend's
> "late" stage of suspending devices and during system resume's "early"
> device resume stage.
>
> Taking all of the above into consideration, make the PM core
> acquire a runtime PM reference to every device and resume it if
> there's a runtime PM resume request pending right before executing
> the subsystem-level .suspend() callback for it.  Make the PM core
> drop references to all devices right after executing the
> subsystem-level .resume() callbacks for them.  Additionally,
> make the PM core disable the runtime PM framework for all devices
> during system suspend, after executing the subsystem-level .suspend()
> callbacks for them, and enable the runtime PM framework for all
> devices during system resume, right before executing the
> subsystem-level .resume() callbacks for them.
>
> Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>

OK, I'm convinced.  :)

Acked-by: Kevin Hilman <khilman@...com>

Minor documentation comment below...

>  Documentation/power/runtime_pm.txt |   20 ++++++++++++++++++++
>  drivers/base/power/main.c          |   27 ++++++++++++++++++++++-----
>  2 files changed, 42 insertions(+), 5 deletions(-)
>
> Index: linux-2.6/drivers/base/power/main.c
> ===================================================================
> --- linux-2.6.orig/drivers/base/power/main.c
> +++ linux-2.6/drivers/base/power/main.c
> @@ -505,6 +505,7 @@ static int legacy_resume(struct device *
>  static int device_resume(struct device *dev, pm_message_t state, bool async)
>  {
>  	int error = 0;
> +	bool put = false;
>  
>  	TRACE_DEVICE(dev);
>  	TRACE_RESUME(0);
> @@ -521,6 +522,9 @@ static int device_resume(struct device *
>  	if (!dev->power.is_suspended)
>  		goto Unlock;
>  
> +	pm_runtime_enable(dev);
> +	put = true;
> +
>  	if (dev->pm_domain) {
>  		pm_dev_dbg(dev, state, "power domain ");
>  		error = pm_op(dev, &dev->pm_domain->ops, state);
> @@ -563,6 +567,10 @@ static int device_resume(struct device *
>  	complete_all(&dev->power.completion);
>  
>  	TRACE_RESUME(error);
> +
> +	if (put)
> +		pm_runtime_put_sync(dev);
> +
>  	return error;
>  }
>  
> @@ -843,16 +851,22 @@ static int __device_suspend(struct devic
>  	int error = 0;
>  
>  	dpm_wait_for_children(dev, async);
> -	device_lock(dev);
>  
>  	if (async_error)
> -		goto Unlock;
> +		return 0;
> +
> +	pm_runtime_get_noresume(dev);
> +	if (pm_runtime_barrier(dev) && device_may_wakeup(dev))
> +		pm_wakeup_event(dev, 0);
>  	if (pm_wakeup_pending()) {
> +		pm_runtime_put_sync(dev);
>  		async_error = -EBUSY;
> -		goto Unlock;
> +		return 0;
>  	}
>  
> +	device_lock(dev);
> +
>  	if (dev->pm_domain) {
>  		pm_dev_dbg(dev, state, "power domain ");
>  		error = pm_op(dev, &dev->pm_domain->ops, state);
> @@ -890,12 +904,15 @@ static int __device_suspend(struct devic
>   End:
>  	dev->power.is_suspended = !error;
>  
> - Unlock:
>  	device_unlock(dev);
>  	complete_all(&dev->power.completion);
>  
> -	if (error)
> +	if (error) {
> +		pm_runtime_put_sync(dev);
>  		async_error = error;
> +	} else if (dev->power.is_suspended) {
> +		__pm_runtime_disable(dev, false);
> +	}
>  
>  	return error;
>  }
> Index: linux-2.6/Documentation/power/runtime_pm.txt
> ===================================================================
> --- linux-2.6.orig/Documentation/power/runtime_pm.txt
> +++ linux-2.6/Documentation/power/runtime_pm.txt
> @@ -567,6 +567,11 @@ this is:
>  	pm_runtime_set_active(dev);
>  	pm_runtime_enable(dev);
>  
> +The PM core always increments the run-time usage counter before calling the
> +->suspend() callback and decrements it after calling the ->resume() callback.
> +Hence disabling run-time PM temporarily like this will not cause any run-time
> +suspend callbacks to be lost.
> +
>  On some systems, however, system sleep is not entered through a global firmware
>  or hardware operation.  Instead, all hardware components are put into low-power
>  states directly by the kernel in a coordinated way.  Then, the system sleep
> @@ -579,6 +584,21 @@ place (in particular, if the system is n
>  be more efficient to leave the devices that had been suspended before the system
>  suspend began in the suspended state.
>  
> +The PM core does its best to reduce the probability of race conditions between
> +the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
> +out the following operations:
> +
> +  * During system suspend it acquires a runtime PM reference to every device
> +    and resume it if there's a runtime PM resume request pending right before

minor: s/resume/resumes/

> +    executing the subsystem-level .suspend() callback for it.  In addition to
> +    that it disables the runtime PM framework for every device right after
> +    executing the subsystem-level .suspend() callback for it.
> +
> +  * During system resume it enables the runtime PM framework for all devices
> +    right before executing the subsystem-level .resume() callbacks for them.
> +    Additionally, it drops references to all devices right after executing the
> +    subsystem-level .resume() callbacks for them.
> +
>  7. Generic subsystem callbacks
>  
>  Subsystems may wish to conserve code space by using the set of generic power

Kevin
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