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SubjectRe: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.
On Wed, 29 Oct 2008, Miklos Szeredi wrote:

> Actually I was thinking of an rw-semaphore, not a mutex. But yeah
> that still has scalability problems. But it could be done with custom
> locking primitives, optimized for this case:
>
> suspend_disable();
> /* driver stuff */
> suspend_enable();

Yes, it could be done. And the overhead could be minimized by using
per-CPU variables. It would still be an awful lot of work, and easy to
get wrong.

> > The problem with unrestricted freezing shows up when you freeze tasks
> > that hold a mutex or other sort of lock. If this mutex is needed later
> > on for suspending a device then the suspend will hang, because a frozen
> > task can't release any mutexes.
>
> I did a random sampling of ->suspend() callbacks, and they don't seem
> to be taking mutexes. Does that happen at all?

It does, particularly among drivers that do runtime PM, which is
becoming more and more important.

Besides, suspend has to synchronize with I/O somehow. Right now that
is handled by making suspend wait until no tasks are doing I/O (because
they are all frozen). If you allow tasks to be frozen at more or less
arbitrary times, while holding arbitrary locks, then you may end up
freezing a task that's in the middle of I/O. That should certainly
block the suspend (not to mention messing up the I/O operation).

> Did anybody ever try modifying the freezer for suspend (not
> hibernate), so that it allows tasks not in running state to freeze?
> If not, I think that's an experiment worth doing.

What happens if the reason the task isn't running is because it's
waiting for I/O to complete? I just don't think this can be made to
work.

Alan Stern



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