linux-kernel - Re: [PATCH v2] PM: runtime: add might

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Date:	Wed, 27 Jul 2011 11:37:21 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Kevin Hilman <khilman@...com>, Nishanth Menon <nm@...com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v2] PM: runtime: add might_sleep to PM runtime functions

On Wednesday, July 27, 2011, Colin Cross wrote:
> On Tue, Jul 26, 2011 at 3:14 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Monday, July 25, 2011, Colin Cross wrote:
> >> Some of the entry points to pm runtime are not safe to
> >> call in atomic context unless pm_runtime_irq_safe() has
> >> been called.  Inspecting the code, it is not immediately
> >> obvious that the functions sleep at all, as they run
> >> inside a spin_lock_irqsave, but under some conditions
> >> they can drop the lock and turn on irqs.
> >>
> >> If a driver incorrectly calls the pm_runtime apis, it can
> >> cause sleeping and irq processing when it expects to stay
> >> in atomic context.
> >>
> >> Add might_sleep_if to all the __pm_runtime_* entry points
> >> to enforce correct usage.
> >>
> >> Add pm_runtime_put_sync_autosuspend to the list of
> >> functions that can be called in atomic context.
> >>
> >> Signed-off-by: Colin Cross <ccross@...roid.com>
> >> ---
> >>  Documentation/power/runtime_pm.txt |    1 +
> >>  drivers/base/power/runtime.c       |   15 ++++++++++++---
> >>  2 files changed, 13 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt
> >> index c291233..1ad507c 100644
> >> --- a/Documentation/power/runtime_pm.txt
> >> +++ b/Documentation/power/runtime_pm.txt
> >> @@ -469,6 +469,7 @@ pm_runtime_resume()
> >>  pm_runtime_get_sync()
> >>  pm_runtime_put_sync()
> >>  pm_runtime_put_sync_suspend()
> >> +pm_runtime_put_sync_autosuspend()
> >>
> >>  5. Run-time PM Initialization, Device Probing and Removal
> >>
> >> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> >> index 2e746f8..f3d8583 100644
> >> --- a/drivers/base/power/runtime.c
> >> +++ b/drivers/base/power/runtime.c
> >> @@ -731,13 +731,16 @@ EXPORT_SYMBOL_GPL(pm_schedule_suspend);
> >>   * return immediately if it is larger than zero.  Then carry out an idle
> >>   * notification, either synchronous or asynchronous.
> >>   *
> >> - * This routine may be called in atomic context if the RPM_ASYNC flag is set.
> >> + * This routine may be called in atomic context if the RPM_ASYNC flag is set,
> >> + * or if pm_runtime_irq_safe() has been called.
> >>   */
> >>  int __pm_runtime_idle(struct device *dev, int rpmflags)
> >>  {
> >>       unsigned long flags;
> >>       int retval;
> >>
> >> +     might_sleep_if(!(rpmflags & RPM_ASYNC) && !dev->power.irq_safe);
> >> +
> >
> > Now that I think of it, perhaps it's better to put the might_sleep()
> > annotations into the actual code paths that should trigger them instead of
> > checking the conditions upfront on every call?  This way we'll avoid quite
> > some overhead that's only necessary for debugging.
> >
> 
> You can't put the might_sleep after the spin_lock_irqsave(), because
> you are always in atomic context, and you can't put it after the
> spin_unlock_irq() that triggers the problem because you have already
> unconditionally left atomic context.
> 
> Anyways, the sleeps happen in a farily rare case, so putting the
> might_sleep in a more specific location will hide the errors when
> developers perform simple tests.  For example, every kmalloc ends up
> calling might_sleep_if(flags & __GFP_WAIT), so that putting
> kmalloc(..., GFP_KERNEL) will print a stack trace every time, instead
> of only the very rare case when kmalloc has to block in a low memory
> condition.
> 
> The calls are very low overhead - the condition in the
> might_sleep_if(), and then in the common case:
> 	if ((preempt_count_equals(preempt_offset) && !irqs_disabled()) || ...)
> 		return;

OK, I'm going to take the $subject patch for 3.2.

Thanks,
Rafael
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