On Mon, Jan 16, 2023 at 09:54:32AM -0800, Boqun Feng wrote: > On Mon, Jan 16, 2023 at 06:36:43PM +0100, Paolo Bonzini wrote: > > On 1/13/23 20:11, Paul E. McKenney wrote: > > > On Fri, Jan 13, 2023 at 10:05:22AM -0800, Boqun Feng wrote: > > > > On Fri, Jan 13, 2023 at 03:29:49AM -0800, Paul E. McKenney wrote: > > > > I prefer that the first two patches go through your tree, because it > > > > reduces the synchronization among locking, rcu and KVM trees to the > > > > synchronization betwen rcu and KVM trees. > > > > > > Very well, I have queued and pushed these with the usual wordsmithing, > > > thank you! > > > > I'm worried about this case: > > > > CPU 0 CPU 1 > > -------------------- ------------------ > > lock A srcu lock B > > srcu lock B lock A > > srcu unlock B unlock A > > unlock A srcu unlock B > > > > While a bit unclean, there is nothing that downright forbids this; as long > > as synchronize_srcu does not happen inside lock A, no deadlock can occur. > > > > First, even with my change, lockdep won't report this as a deadlock > because srcu_read_lock() is annotated as a recursive (unfair) read lock > (the "read" parameter for lock_acquire() is 2) and in this case lockdep > knows that it won't cause deadlock. > > For SRCU, the annotation mapping that is 1) srcu_read_lock() is marked > as recursive read lock and 2) synchronize_srcu() is marked as write lock > sync, recursive read locks themselves cannot cause deadlocks and lockdep > is aware of it. > > Will add a selftest for this later. > > > However, if srcu is replaced with an rwlock then lockdep should and does > > report a deadlock. Boqun, do you get a false positive or do your patches > > To be more precise, to have a deadlock, the read lock on CPU 0 has to be > a *fair* read lock (i.e. non-recursive reader, the "read" parameter for > lock_acquire is 1) > > > correctly suppress this? > > I'm pretty sure that lockdep handles this ;-) And lockdep agrees, refusing to complain about the following: idx = srcu_read_lock(&srcu1); mutex_lock(&mut1); mutex_unlock(&mut1); srcu_read_unlock(&srcu1, idx); mutex_lock(&mut1); idx = srcu_read_lock(&srcu1); srcu_read_unlock(&srcu1, idx); mutex_unlock(&mut1); Thanx, Paul
