On 10/02/2012 10:03 AM, Daniel Vetter wrote: > On Tue, Oct 02, 2012 at 08:46:32AM +0200, Thomas Hellstrom wrote: >> On 10/01/2012 11:47 AM, Maarten Lankhorst wrote: >>> I was doing a evil hack where I 'released' lru_lock to lockdep before doing >>> the annotation >>> for a blocking acquire, and left trylock annotations as they were. This >>> made lockdep do the >>> right thing. >> I've never looked into how lockdep works. Is this something that can >> be done permanently or just for testing >> purposes? Although not related to this, is it possible to do >> something similar to the trylock reversal in the >> fault() code where mmap_sem() and reserve() change order using a >> reserve trylock? > lockdep just requires a bunch of annotations, is a compile-time configure > option CONFIG_PROVE_LOCKING and if disabled, has zero overhead. And it's > rather awesome in detected deadlocks and handling crazy locking schemes > correctly: > - correctly handles trylocks > - correctly handles nested locking (i.e. grabbing a global lock, then > grabbing subordinate locks in an unordered sequence since the global > lock ensures that no deadlocks can happen). > - any kinds of inversions with special contexts like hardirq, softirq > - same for page-reclaim, i.e. it will yell if you could (potentially) > deadlock because your shrinker grabs a lock that you hold while calling > kmalloc. > - there are special annotates for various subsystems, e.g. to check for > del_timer_sync vs. locks held by that timer. Or the console_lock > annotations I've just recently submitted. > - all that with a really flexible set of annotation primitives that afaics > should work for almost any insane locking scheme. The fact that Maarten > could come up with proper reservation annotations without any changes to > lockdep testifies this (he only had to fix a tiny thing to make it a bit > more strict in a corner case). > > In short I think it's made of awesome. The only downside is that it lacks > documentation, you have to read the code to understand it :( > > The reason I've suggested to Maarten to abolish the trylock_reservation > within the lru_lock is that in that way lockdep only ever sees the > trylock, and hence is less strict about complainig about deadlocks. But > semantically it's an unconditional reserve. Maarten had some horrible > hacks that leaked the lockdep annotations out of the new reservation code, > which allowed ttm to be properly annotated. But those also reduced the > usefulness for any other users of the reservation code, and so Maarten > looked into whether he could remove that trylock dance in ttm. > > Imo having excellent lockdep support for cross-device reservations is a > requirment, and ending up with less strict annotations for either ttm > based drivers or other drivers is not good. And imo the ugly layering that > Maarten had in his first proof-of-concept also indicates that something is > amiss in the design. > > 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?). /Thomas
