On Fri, 13 Jan 2023 23:32:01 -0800 Boqun Feng <boqun.feng@xxxxxxxxx> > On Sat, Jan 14, 2023 at 03:18:32PM +0800, Hillf Danton wrote: > > > > task X task Y > > --- --- > > mutex_lock(A); > > srcu_read_lock(B); > > srcu_lock_acquire(&B->dep_map); > > a) lock_map_acquire_read(&B->dep_map); > > synchronze_srcu(B); > > __synchronize_srcu(B); > > srcu_lock_sync(&B->dep_map); > > lock_map_sync(&B->dep_map); > > lock_sync(&B->dep_map); > > __lock_acquire(&B->dep_map); > > At this time, lockdep add dependency A -> B in the dependency graph. > > > b) lock_map_acquire_read(&B->dep_map); > > __lock_release(&B->dep_map); > > c) lock_map_acquire_read(&B->dep_map); > > mutex_lock(A); > > and here, lockdep will try to add dependency B -> A into the dependency > graph, and find that A -> B -> A will form a circle (with strong > dependency), therefore lockdep knows it's a deadlock. Is the strong dependency applying to mode c)? If yes then deadlock should be also detected in the following locking pattern that has no deadlock. cpu0 cpu1 --- --- mutex_lock A mutex_lock B mutex_unlock B mutex_lock B mutex_lock A > > > > > No deadlock could be detected if taskY takes mutexA after taskX releases B, > > The timing that taskX releases B doesn't master, since lockdep uses > graph to detect deadlocks rather than after-fact detection. > > > and how taskY acquires B does not matter as per the a), b) and c) modes in > > the above chart, again because releasing lock can break deadlock in general. > > I have test cases showing the above deadlock can be detected, so if you > think there is a deadlock that may dodge from my change, feel free to > add a test case in lib/locking-selftest.c ;-) > > Regards, > Boqun
