Re: [linux-pm] [PATCH/RFC] MMC: remove unbalanced pm_runtime_suspend()

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, April 25, 2011, Rafael J. Wysocki wrote:
> On Saturday, April 23, 2011, Rafael J. Wysocki wrote:
> > On Friday, April 22, 2011, Alan Stern wrote:
> > > On Fri, 22 Apr 2011, Rafael J. Wysocki wrote:
> > > 
> > > > > The barrier would not prevent the race between the notifier and runtie PM
> > > > > from taking place.  Why don't we do something like this instead:
> > > > > 
> > > > > ---
> > > > >  drivers/base/dd.c |    3 ++-
> > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > 
> > > > > Index: linux-2.6/drivers/base/dd.c
> > > > > ===================================================================
> > > > > --- linux-2.6.orig/drivers/base/dd.c
> > > > > +++ linux-2.6/drivers/base/dd.c
> > > > > @@ -326,6 +326,8 @@ static void __device_release_driver(stru
> > > > >  						     BUS_NOTIFY_UNBIND_DRIVER,
> > > > >  						     dev);
> > > > >  
> > > > > +		pm_runtime_put_sync(dev);
> > > > > +
> > > > 
> > > > In fact, I think this one may be _noidle.  If we allow the bus/driver
> > > > to do what they wont, we might as well let them handle the "device idle"
> > > > case from ->remove().
> > > 
> > > Maybe...  But keeping it put_sync doesn't do any harm.  In Guennadi's 
> > > case, it might allow him to get rid of the pm_runtime_suspend() call in 
> > > the remove routine.
> > > 
> > > > >  		if (dev->bus && dev->bus->remove)
> > > > >  			dev->bus->remove(dev);
> > > > >  		else if (drv->remove)
> > > > > @@ -338,7 +340,6 @@ static void __device_release_driver(stru
> > > > >  						     BUS_NOTIFY_UNBOUND_DRIVER,
> > > > >  						     dev);
> > > > >  
> > > > > -		pm_runtime_put_sync(dev);
> > > > >  	}
> > > > >  }
> > > 
> > > Basically this is okay with me, and it should allow Guennadi to avoid
> > > the extra put/get pair.
> > 
> > OK, so I'm going to put the appended patch into my linux-next branch
> > (hopefully, the problem is explained sufficiently in the changelog).
> 
> I thought about that a bit more and came to the conclusion that we should
> do things a bit differently in __device_release_driver().  Namely, the fact
> that the device can be resumed (either synchronously or asynchronously) after
> the pm_runtime_barrier() has returned may be problematic too, because it
> may race with the bus notifier in some cases.  For this reason, I think it
> would be better to do pm_runtime_get_sync() instead of the
> pm_runtime_get_noresume() and pm_runtime_barrier().
> 
> So, I think the appended patch would be better than the previous one.

Well, there haven't been any objections, so I'm adding the patch below
to my linux-next branch.

Thanks,
Rafael


> ---
> From: Rafael J. Wysocki <rjw@sisk.pl>
> Subject: PM / Runtime: Rework runtime PM handling during driver removal
> 
> The driver core tries to prevent race conditions between runtime PM
> and driver removal from happening by incrementing the runtime PM
> usage counter of the device and executing pm_runtime_barrier() before
> running the bus notifier and the ->remove() callbacks provided by the
> device's subsystem or driver.  This guarantees that, if a future
> runtime suspend of the device has been scheduled or a runtime resume
> or idle request has been queued up right before the driver removal,
> it will be canceled or waited for to complete and no other
> asynchronous runtime suspend or idle requests for the device will be
> put into the PM workqueue until the ->remove() callback returns.
> However, it doesn't prevent resume requests from being queued up
> after pm_runtime_barrier() has been called and it doesn't prevent
> pm_runtime_resume() from executing the device subsystem's runtime
> resume callback.  Morever, it prevents the device's subsystem or
> driver from putting the device into the suspended state by calling
> pm_runtime_suspend() from its ->remove() routine.  This turns out to
> be a major inconvenience for some subsystems and drivers that want to
> leave the devices they handle in the suspended state.
> 
> To really prevent runtime PM callbacks from racing with the bus
> notifier callback in __device_release_driver(), which is necessary,
> because the notifier is used by some subsystems to carry out
> operations affecting the runtime PM functionality, use
> pm_runtime_get_sync() instead of the combination of
> pm_runtime_get_noresume() and pm_runtime_barrier().  This will resume
> the device if it's in the suspended state and will prevent it from
> being suspended again until pm_runtime_put_*() is called.
> 
> To allow subsystems and drivers to put devices into the suspended
> state by calling pm_runtime_suspend() from their ->remove() routines,
> execute pm_runtime_put_sync() after running the bus notifier in
> __device_release_driver().  This will require subsystems and drivers
> to make their ->remove() callbacks avoid races with runtime PM
> directly, but it will allow of more flexibility in the handling of
> devices during the removal of their drivers.
> 
> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
> ---
>  drivers/base/dd.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6/drivers/base/dd.c
> ===================================================================
> --- linux-2.6.orig/drivers/base/dd.c
> +++ linux-2.6/drivers/base/dd.c
> @@ -316,8 +316,7 @@ static void __device_release_driver(stru
>  
>  	drv = dev->driver;
>  	if (drv) {
> -		pm_runtime_get_noresume(dev);
> -		pm_runtime_barrier(dev);
> +		pm_runtime_get_sync(dev);
>  
>  		driver_sysfs_remove(dev);
>  
> @@ -326,6 +325,8 @@ static void __device_release_driver(stru
>  						     BUS_NOTIFY_UNBIND_DRIVER,
>  						     dev);
>  
> +		pm_runtime_put_sync(dev);
> +
>  		if (dev->bus && dev->bus->remove)
>  			dev->bus->remove(dev);
>  		else if (drv->remove)
> @@ -338,7 +339,6 @@ static void __device_release_driver(stru
>  						     BUS_NOTIFY_UNBOUND_DRIVER,
>  						     dev);
>  
> -		pm_runtime_put_sync(dev);
>  	}
>  }
>