On Tue, Dec 01, 2020 at 05:26:32PM -0500, Joel Fernandes wrote: > On Thu, Nov 19, 2020 at 3:42 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, Nov 19, 2020 at 12:16:15PM -0800, Paul E. McKenney wrote: > > > On Thu, Nov 19, 2020 at 02:44:35PM -0500, Joel Fernandes wrote: > > > > On Thu, Nov 19, 2020 at 2:22 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > > > > On Wed, Nov 18, 2020 at 11:15:41AM -0500, Joel Fernandes (Google) wrote: > > > > > > > > > > After rcu_do_batch(), add a check for whether the seglen counts went to > > > > > > > > > > zero if the list was indeed empty. > > > > > > > > > > > > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > > > > > > > > > > > > > > > > > Queued for testing and further review, thank you! > > > > > > > > > > > > > > > > FYI, the second of the two checks triggered in all four one-hour runs of > > > > > > > > TREE01, all four one-hour runs of TREE04, and one of the four one-hour > > > > > > > > runs of TREE07. This one: > > > > > > > > > > > > > > > > WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0); > > > > > > > > > > > > > > > > That is, there are callbacks in the list, but the sum of the segment > > > > > > > > counts is nevertheless zero. The ->nocb_lock is held. > > > > > > > > > > > > > > > > Thoughts? > > > > > > > > > > > > > > FWIW, TREE01 reproduces it very quickly compared to the other two > > > > > > > scenarios, on all four run, within five minutes. > > > > > > > > > > > > So far for TREE01, I traced it down to an rcu_barrier happening so it could > > > > > > be related to some interaction with rcu_barrier() (Just a guess). > > > > > > > > > > Well, rcu_barrier() and srcu_barrier() are the only users of > > > > > rcu_segcblist_entrain(), if that helps. Your modification to that > > > > > function looks plausible to me, but the system's opinion always overrules > > > > > mine. ;-) > > > > > > > > Right. Does anything the bypass code standout? That happens during > > > > rcu_barrier() as well, and it messes with the lengths. > > > > > > In theory, rcu_barrier_func() flushes the bypass before doing the > > > entrain, and does the rcu_segcblist_entrain() afterwards. > > > > > > Ah, and that is the issue. If ->cblist is empty and ->nocb_bypass > > > is not, then ->cblist length will be nonzero, and none of the > > > segments will be nonzero. > > > > > > So you need something like this for that second WARN, correct? > > > > > > WARN_ON_ONCE(!rcu_segcblist_empty(&rdp->cblist) && > > > rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0); > > Just started to look into it again. If the &rdp->cblist is empty, that > means the bypass list could not have been used (Since per comments on > rcu_nocb_try_bypass() , the bypass list is in use only when the cblist > is non-empty). So the cblist was non empty, then the segment counts > should not sum to 0. So I don't think that explains it. Anyway, I > will try the new version of your warning in case there is something > about bypass lists that I'm missing. Good point. I really did see failures, though. Do they show up for you? Thanx, Paul
