On Wed, Nov 06, 2024 at 12:18:25PM -0800, Paul E. McKenney wrote: > Good eyes!!! > > But did you find this with KCSAN, or by visual inspection? > > The reason that I ask is that the __note_gp_changes() should be > invoked with the leaf rnp->lock held, which should exclude writes to > the rdp->gpwrap fields for all CPUs corresponding to that leaf rcu_node > structure. > > Note the raw_lockdep_assert_held_rcu_node(rnp) call at the beginning of > this function. > > So I believe that the proper fix is to *remove* READ_ONCE() from accesses > to rdp->gpwrap in this function. > > Or am I missing something here? > > Thanx, Paul I found this by visual inspection. When reviewing the function __note_gp_changes(), I noticed that other accesses to rdp->gpwrap are protected with either READ_ONCE() or WRITE_ONCE(), which led me to suspect a potential data race at line 1305. However, I am not certain whether holding rnp->lock protects access to rdp->gpwrap in this case. If it indeed ensures that no concurrent writes can occur, then I agree that the correct approach would be to remove READ_ONCE() from those accesses. Thanks, Zilin
