Messages in this thread | | | Date | Tue, 20 Oct 2015 10:40:12 -0400 | From | Jerome Glisse <> | Subject | Re: [PATCH] locking/lockdep: Fix expected depth value in __lock_release() |
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On Tue, Oct 20, 2015 at 03:07:29PM +0200, Peter Zijlstra wrote: > On Tue, Oct 20, 2015 at 08:42:19AM -0400, Jerome Glisse wrote: > > On Tue, Oct 20, 2015 at 02:18:53PM +0200, Peter Zijlstra wrote: > > > > What code did you find that triggered this? That is, what code is taking > > > nested locks with other locks in the middle? (Not wrong per-se, just > > > curious how that would come about). > > > > Well i am not able to reproduce myself but it happens as part of > > mm_drop_all_locks() as to which lock does trigger i am unsure as > > all the i_mmap_rwsem are taken one after the other and same for > > anon_vma rwsem so they should already coalesce properly. My guess > > is that code calling all lock also have a mutex and once all vma > > lock are drop the mutex coalesce with mm_all_locks_mutex. > > Ah yes, mm_all_locks_mutex looks like a likely candidate. > > Curious, this code is ancient, and I've never seen a report of this > triggering. > > > > > This patch adjust the expect depth value by decrementing it if > > > > what was previously 2 entry inside the stack are coalesced into > > > > only one entry. > > > > > > Would it not make more sense to scan the entire hlock stack on > > > __lock_acquire() and avoid getting collapsible entries in the first > > > place? > > > > > > Something like so... > > > > It would work too, probably more compute intensive than my solution > > but this is lockdep code so i guess it is fine. Also dunno if we loose > > any valuable information by not keeping the stack ordered so one > > can check order in whick lock are taken. > > Right; its a little bit more expensive, but only for acquires with > nest_lock set, which should be rare. > > As to the order; since they're all of the same class, its fine to > collapse them. > > However the proposed alternative avoids 'strange' boundary cases like: > > mutex_lock(&top_lock); > > for (...) { > mutex_lock_nest_lock(&obj->lock1, &top_lock); > mutex_lock_nest_lock(&obj->lock2, &top_lock); > } > > Which would currently result in running our of lock stack space real > quick since it would never be able to collapse. > > In any case, can you 'test' the proposed alternative in any way?
I will ask for it to be tested probably gonna take couple days before i hear back. I will report as soon as i have confirmation.
Cheers, Jérôme
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