On Sun, Nov 11, 2018 at 06:24:55PM -0800, Paul E. McKenney wrote: > > > There were quite a few commits involved in making this happen. Perhaps > > > the most pertinent are these: > > > > > > 3e3100989869 ("rcu: Defer reporting RCU-preempt quiescent states when > > > disabled") > > > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU > > > PREEMPT builds") > > > > The latter; it does not mention that this will possible make > > synchronize_sched() quite a bit more expensive on PREEMPT=y builds :/ > > In theory, sure. In practice, people have switched any number of > things from RCU-sched to RCU and back without problems.
Still, better safe than sorry. It was a rather big change in behaviour, so it wouldn't have been strange to call that out. > > But for PREEMPT=y synchronize_sched() can be quite a bit shorter than > > synchronize_rcu(), since we don't have to wait for preempted read side > > stuff. > > Again, there are quite a few places that have managed that transition > without issue. Why do you expect this change to have problems that have > not been seen elsewhere? I'm not, I'm just taking issue with the Changelog. > > Again, the patch didn't say that. > > > > If the Changelog would've read something like: > > > > "Since synchronize_sched() is now equivalent to synchronize_rcu(), > > replace the synchronize_sched() usage such that we can eventually remove > > the interface." > > > > It would've been clear that the patch is a nop and what the purpose > > was. > > I can easily make that change. Please, sufficient doesn't imply necessary etc.. A changelog should always clarify why we do the patch.