Messages in this thread | | | Date | Sun, 11 Apr 2010 00:12:06 +0900 | From | Hitoshi Mitake <> | Subject | Re: Question about lock sequence |
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On 04/10/10 22:07, Frederic Weisbecker wrote: > On Sat, Apr 10, 2010 at 07:44:07PM +0900, Hitoshi Mitake wrote: >> >> Hi, >> >> I found that my understand about lockdep is completely wrong :( , >> so state machine of perf lock should be fixed before optimization. >> >> And I found that behaviour related to some of spin locks are strange. >> The concrete example is lock sequences targeting dcache_lock (defined in >> head of fs/dcache.c). >> >> I made a little (and not essential) change to perf lock, and observe >> lock sequence targeting it. >> Changed perf lock shows sequence of locks in time order, >> and I grepped the output of it with dcache, like this: >> >> % sudo ./perf lock report | grep dcache >> >> The head part of result is this: >> #<name>-<pid> <time (in u64)> <action> <address of lockdep> <name of lock> >> perf-3238 92430534170 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92430536714 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92431444481 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92431446061 acquired: 0xffffffff81a4b398 dcache_lock >> perf-3238 92431448157 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92431449670 acquired: 0xffffffff81a4b398 dcache_lock >> perf-3238 92432371136 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92432372712 acquired: 0xffffffff81a4b398 dcache_lock >> perf-3238 92432374718 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92432376173 acquired: 0xffffffff81a4b398 dcache_lock >> perf-3238 92433315563 acquire: 0xffffffff81a4b398 dcache_lock >> perf-3238 92433317173 acquired: 0xffffffff81a4b398 dcache_lock >> >> There are too many acquire and acquired without corresponding release >> (or contended). >> If dcache_lock is rwlock and these acquires mean read locks, this is not >> so strange. >> But, for me, this is a pattern of dead lock. >> Of course perf lock finished its work, so there is no actual dead lock. >> >> If you know something about this behaviour of lock, could you tell me? > > > If you can see nesting acquires on an rwlock, it's normal, because it can > be recursively acquired. > > What wouldn't be normal is an unbalanced stacking of acquire - release. > > If you see: > > acquire > acquire > acquire > > You should find the symetric releases: > > release > release > release
Yes. All acquire must have their corresponding release. According to my result, some of ordinary spin locks make the not symmetric pattern.
> > Otherwise we have something wrong.
I'm considering it. But I cannot find something wrong now :( There should be some cause...
> > Also I wonder about the fact you seem to have acquire without acquired > in your trace.
Yeah, if acquire is trylock, it isn't strange. But these acquires are not trylock.
> > I'm going to look at this, hopefully I'll survive after looking in all these > rwlock_* _rwlock_* __rwlock_* arch_* raw* _raw* __raw* do_raw* mess... :) > >
Oh thanks :) I'll continue my quest the cause. Hitoshi
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