On Mon, Nov 12, 2018 at 05:28:52AM -0800, Paul E. McKenney wrote: > On Mon, Nov 12, 2018 at 10:00:47AM +0100, Peter Zijlstra wrote:
> > Still, better safe than sorry. It was a rather big change in behaviour, > > so it wouldn't have been strange to call that out. > > This guy: > > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT > builds") > > Has a commit log that says: > > Now that RCU-preempt knows about preemption disabling, its > implementation of synchronize_rcu() works for synchronize_sched(), > and likewise for the other RCU-sched update-side API members. > This commit therefore confines the RCU-sched update-side code > to CONFIG_PREEMPT=n builds, and defines RCU-sched's update-side > API members in terms of those of RCU-preempt. > > That last phrase seems pretty explicit. What am I missing here? That does not explicitly state that because RCU-preempt synchornize_rcu() can take _much_ longer, the new synchronize_sched() can now take _much_ longer too. So when someone bisects a problem to this commit; and he reads the Changelog, he might get the impression that was unexpected. > Not that it matters, given that I know of no way to change a mainlined > commit log. I suppose I could ask Jon if he would be willing to take > a 2018 RCU API LWN article, if that would help. Yes, it is water under the bridge; but Changelogs should be explicit about behavioural changes. And while the merged RCU has the semantic behaviour required, the timing behaviour did change significantly. > > > > 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. > > ??? Did you mean to say "necessary doesn't imply sufficient"? If so, > what else do you feel is missing? No, I meant to say that your original Changelog only states that sync_rcu now covers rcu-sched behaviour. Which means that the change is sufficient. It completely and utterly fails to explain _why_ you're doing the change. Ie. you do not address why it is necessary. A Changelog should always explain why the change is needed. In this case because you want to get rid of the sync_sched() api.