On Mon, Feb 13, 2023 at 09:03:14PM -0500, Alan Stern wrote: > On Mon, Feb 13, 2023 at 05:51:11PM -0800, Boqun Feng wrote: > > 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); > > Why isn't this treated as a deadlock? It sure looks like a deadlock to > me. Is this an example where lockdep just doesn't get the right answer? > Because A serializes B and C, so that particular piece of code doesn't cause deadlock. Note that you can still use you normal mutex_lock() for B, so if there is more code: T3: mutex_lock(C); mutex_lock(B); lockdep will report deadlock. Regards, Boqun > Alan Stern
