On Fri, Mar 13, 2020 at 01:18:30PM -0700, Linus Torvalds wrote: > On Fri, Mar 13, 2020 at 10:50 AM Jens Axboe <axboe@xxxxxx> wrote: > > > > Just a single fix here, improving the RCU callback ordering from last > > week. After a bit more perusing by Paul, he poked a hole in the > > original. > > Ouch. > > If I read this patch correctly, you're now adding a rcu_barrier() onto > the system workqueue for each io_uring context freeing op. > > This makes me worry: > > - I think system_wq is unordered, so does it even guarantee that the > rcu_barrier happens after whatever work you're expecting it to be > after? > > Or is it using a workqueue not because it wants to serialize with any > other work, but because it needs to use rcu_barrier in a context where > it can't sleep? > > But the commit message does seem to imply that ordering is important.. > > - doesn't this have the potential to flood the system_wq be full of > flushing things that all could take a while.. > > I've pulled it, and it may all be correct, just chalk this message up > to "Linus got nervous looking at it". > > Added Paul and Tejun to the participants explicitly. The idea is that rcu_barrier() waits for callbacks from all earlier call_rcu()s to be invoked. So as long as you know that the call_rcu() happened earlier than the rcu_barrier(), the rcu_barrier() is guaranteed to wait for that call_rcu()'s callback. In this case (and Jens will correct me in the sadly likely event that I get the story confused), we have a call_rcu() followed by scheduling work on that same task. The work has to start executing after it was scheduled, so if that work does an rcu_barrier(), then that rcu_barrier() will wait on the call_rcu()'s callback to be invoked. Jens could invoke the rcu_barrier() just before scheduling the work, but the synchronous delay from the rcu_barrier() is a problem. Jens, what did I mess up in the above story? ;-) I defer to Jens and Tejun on the possibility of ending up with all workqueue kthreads waiting on rcu_barrier(). If that is a problem, there are some ways of dealing with it, though none that I can think of that come for free. Thanx, Paul
