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

[Alan Stern]

This idea just occurred to me.  It may or may not end up being useful, 
and I don't have any specific applications in mind yet.

System sleeps are supposed to be transparent to userspace.  An I/O 
request submitted just before the sleep starts should be blocked until 
after the system wakes up.

Without the freezer to do this blocking for them, drivers will have to 
do it themselves.  So when a driver is about to carry out an I/O 
operation, it has to insure that the operation is mutually exclusive 
with sleeping.

One way to do this is by a private mutex, which would protect both the
block of code doing the I/O and the suspend routine.  One for each
device; that's a lot of new mutexes.

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?

Alan Stern