On 10/03/2012 02:46 PM, Maarten Lankhorst wrote: > Op 03-10-12 12:53, Thomas Hellstrom schreef: >> On 10/03/2012 10:53 AM, Daniel Vetter wrote: >>> On Wed, Oct 3, 2012 at 10:37 AM, Thomas Hellstrom <thellstrom at >>> vmware.com> wrote: >>>>>> So if I understand you correctly, the reservation changes in TTM are >>>>>> motivated by the >>>>>> fact that otherwise, in the generic reservation code, lockdep can only be >>>>>> annotated for a trylock and not a waiting lock, when it *is* in fact a >>>>>> waiting lock. >>>>>> >>>>>> I'm completely unfamiliar with setting up lockdep annotations, but the >>>>>> only >>>>>> place a >>>>>> deadlock might occur is if the trylock fails and we do a >>>>>> wait_for_unreserve(). >>>>>> Isn't it possible to annotate the call to wait_for_unreserve() just like >>>>>> an >>>>>> interruptible waiting lock >>>>>> (that is always interrupted, but at least any deadlock will be catched?). >>>>> Hm, I have to admit that idea hasn't crossed my mind, but it's indeed >>>>> a hole in our current reservation lockdep annotations - since we're >>>>> blocking for the unreserve, other threads could potential block >>>>> waiting on us to release a lock we're holding already, resulting in a >>>>> deadlock. >>>>> >>>>> Since no other locking primitive that I know of has this >>>>> wait_for_unlocked interface, I don't know how we could map this in >>>>> lockdep. One idea is to grab the lock and release it again immediately >>>>> (only in the annotations, not the real lock ofc). But I need to check >>>>> the lockdep code to see whether that doesn't trip it up. >>>> I imagine doing the same as mutex_lock_interruptible() does in the >>>> interrupted path should work... >>> It simply calls the unlock lockdep annotation function if it breaks >>> out. So doing a lock/unlock cycle in wait_unreserve should do what we >>> want. >>> >>> And to properly annotate the ttm reserve paths we could just add an >>> unconditional wait_unreserve call at the beginning like you suggested >>> (maybe with #ifdef CONFIG_PROVE_LOCKING in case ppl freak out about >>> the added atomic read in the uncontended case). >>> -Daniel >> I think atomic_read()s are cheap, at least on intel as IIRC they don't >> require bus locking, >> still I think we should keep it within CONFIG_PROVE_LOCKING >> >> which btw reminds me there's an optimization that can be done in that one >> should really only >> call atomic_cmpxchg() if a preceding atomic_read() hints that it will >> succeed. >> >> Now, does this mean TTM can keep the atomic reserve <-> lru list removal? > I don't think it would be a good idea to keep this across devices, Why?
> there's currently no > callback to remove buffers off the lru list. So why don't we add one, and one to put them on the *correct* LRU list while unreserving. /Thomas
