On 06/08/09 11:58 +0200, Louis Rilling wrote: > On 05/08/09 17:01 -0700, Benjamin Blum wrote: > > On Wed, Aug 5, 2009 at 9:42 AM, Louis Rilling<Louis.Rilling@xxxxxxxxxxx> wrote: > > > On 05/08/09 9:11 -0700, Paul Menage wrote: > > >> On Wed, Aug 5, 2009 at 3:20 AM, Louis Rilling<Louis.Rilling@xxxxxxxxxxx> wrote: > > >> > > > >> > The downside of this is teaching lockdep about this recursive locking. Not that > > >> > simple actually... > > >> > > >> Don't we just give each thread's lock its own lock class? That's what > > >> we did for the cgroup hierarchy_mutex. > > > > > > Given that lock classes must be static and that lockdep only supports a limited > > > lock depth, this is an issue for processes having many threads. > > > > > >> > > >> > so that such cases are currently handled using a higher-level > > >> > lock that prevents races in locking the whole chain (there was one such example > > >> > for locking all vmas with KVM). IIUC, the intent here is to avoid such > > >> > higher-level lock. > > >> > > >> cgroup_mutex already fulfills the role of the higher-level lock. > > > > > > If so (that is, here cgroup_mutex is taken before write-locking all threads' > > > rw_sem), then enhancing rwsem's interface in a similar way to the > > > spin_lock_nest_lock() interface could do it. There will still be an issue with > > > many threads and lockdep limited lock depth though. > > > > If we make the locks per-thread, then we can use plain mutexes instead > > of rwsems since the only reader will ever be the owning thread itself, > > and we can use mutex_lock_nested. > > mutex_lock_nested is not enough, since this would require putting each thread's > mutex in a different class. Not exactly what I meant, sorry. This would require defining as many sub-classes as possible (still limited to MAX_LOCK_DEPTH) to support many threads. > Again, something like mutex_lock_nest_lock() is > the solution, especially since Peter's recent improvement. > Louis -- Dr Louis Rilling Kerlabs Skype: louis.rilling Batiment Germanium Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes http://www.kerlabs.com/ 35700 Rennes
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