On Sat, Jan 14, 2023 at 06:26:59PM +0800, Hillf Danton wrote: > 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 Well, of course, this is how lockdep works. Lockdep detects the *potential* deadlocks rather than detects the deadlocks when they really happen. Otherwise lockdep is useless. The execution in your example shows the potential deadlocks, i.e. one task acquires A and then acquires B, the other task acquires B and then acquires A. Potential deadlocks mean given a correct timing, a deadlock may happen. Regards, Boqun > > > > > > > > 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
