On Saturday, 7 July 2007 04:44, Benjamin Herrenschmidt wrote: > On Fri, 2007-07-06 at 11:31 +0200, Oliver Neukum wrote: > > Am Freitag, 6. Juli 2007 schrieb Benjamin Herrenschmidt: > > > On Fri, 2007-07-06 at 09:13 +0200, Rafael J. Wysocki wrote: > > > > > > > > The only reason (I know of) why we don't handle uninterruptible tasks in the > > > > freezer is that we're afraid of the suspend process deadlocking with an > > > > uninterruptible task holding a lock, but AFAICS the probability of such an > > > > event is extremely small. > > > > > > What would deadlock specifically ? One of the drivers trying to acquire > > > that lock ? It would be a driver bug then. > > > > Your driver's write method looks like: > > > > mutex_lock(); > > poke_some_hardware(); > > wait_event_uninterruptible(); //for result > > res = evaluate_result(); > > mutex_unlock(); > > return res; > > > > If you put a task into the refrigerator at wait_event_interruptible() > > you will deadlock if you need this lock for the driver to go to suspend. > > The suspend method then must not take the lock _and_ it must be > > aware that there may be an ongoing operation. > > Well... 2 things here. Either you have a freezer in which case the > chances of the above scenario are increased, How so? :-) > or you don't, in which case > your suspend method will just sleep on the lock until outstanding HW > accesses that have that lock are completed, and everything is fine. > > You need to be careful with one thing though, whether you have a freezer > or not. If you driver, in some code path, whatever it is (ioctl, kernel > thread, workqueue, ...) does something like: > > mutex_lock > kmalloc(...,GFP_KERNEL); > mutex_unlock > > And it's suspend callback then does: > > mutex_lock > > The problem here is that the disks might already have been suspended > prior to your driver being called. Thus, any attempt at pushing things > out to swap or dirty mmap'ings back to storage will hang, thus kmalloc > can potentially hang (afaik), and you will deadlock. > > That's what I've been talking about earlier when I said that we should > have some security in SLAB/SLUB/Buddy allocators, to silently turn > GFP_KERNEL to at least GFP_NOIO or even ATOMIC before we start > suspending drivers. > > Now, another way to deal with that would have to use > pre-suspend/post-resume notifications, and have drivers avoid doing the > above between those, but that's much harder. (Essentially, drivers would > have to either make sure they don't do things like blocking allocations, > even implicitely, or possibly fall back to a degraded synchronous mode > or that sort of thing). > > I think it's much simpler to tweak slab/slub/buddy instead :-) > > Note that the above issue is orthogonal to our freezer discussion, it's > just one of the potential deadlock cause we have with suspend that needs > to be fixed. Agreed. Greetings, Rafael -- "Premature optimization is the root of all evil." - Donald Knuth _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
