[linux-pm] Helping drivers to work without the freezer

[Alan Stern]

On Sat, 8 Mar 2008, Rafael J. Wysocki wrote:

> > My idea is instead to have the PM core provide a new pair of routines
> > for use by drivers.  Something like "thread_not_sleepable()" and 
> > "thread_sleepable()".  
> > 
> > The first routine would be called by a driver before starting to do
> > I/O, while no locks are held.  If a sleep transition had already
> > started, the routine would block until the sleep was over.  Otherwise,
> > the thread would be marked with a NOT_SLEEPABLE flag until the second
> > routine was called.  When the PM core wanted to start a system sleep
> > it would have to check whether any threads were marked NOT_SLEEPABLE,
> > and wait until none of them were.
> > 
> > This could make drivers a little simpler.  It would mean less code to
> > modify, and it would remove one entry from the messy I/O vs. unbind vs.
> > suspend synchronization problem.
> > 
> > Comments?
> 
> Well, this is what the current freezer does with respect to kernel threads,
> only the name of the flag is different. ;-)

They aren't exactly the same, although they certainly are similar.  The 
difference lies in what happens when a task calls set_freezable() 
after a system sleep has begun; its TIF_FREEZE flag doesn't immediately 
get set.

Also, the current freezer doesn't offer a clear_freezable() routine.

> You basically need something very similar to the current freezer in order
> to implement the "PM core would have to check whether any threads were marked
> NOT_SLEEPABLE, and wait until none of them were" functionality.

Another approach would be to use something like an rwsem.  Hopefully 
without all the cache-line-bouncing overhead on SMP systems.

Alan Stern