On Mon, Feb 13, 2023 at 05:29:49PM +0100, Peter Zijlstra wrote: > On Mon, Feb 13, 2023 at 10:25:59AM -0500, Alan Stern wrote: > > On Mon, Feb 13, 2023 at 10:24:13AM +0100, Peter Zijlstra wrote: > > > On Sun, Feb 12, 2023 at 10:23:44AM -0500, Alan Stern wrote: > > > > Provided it acquires the parent device's lock first, this is > > > > utterly safe no matter what order the children are locked in. Try > > > > telling that to lockdep! > > > > > > mutex_lock_next_lock(child->lock, parent->lock) is there to express this > > > exact pattern, it allows taking multiple child->lock class locks (in any > > > order) provided parent->lock is held. > > > > Ah, this is news to me. Is this sort of thing documented somewhere? Basically if you have two lock instances A and B with the same class, and you know that locking ordering is always A -> B, then you can do mutex_lock(A); mutex_lock_nest_lock(B, A); // lock B. to tell the lockdep this is not deadlock, plus lockdep will treat the acquisition of A and the precondition of acquisition B, so the following is not a deadlock as well: T1: mutex_lock(A); mutex_lock(C); mutex_lock_nest_lock(B, A); T2: mutex_lock(A); mutex_lock_nest_lock(B, A); mutex_lock(C); Regards, Boqun > > Probably not :/
