On Sun, Nov 11, 2018 at 04:45:28PM -0800, Paul E. McKenney wrote: > On Mon, Nov 12, 2018 at 01:12:33AM +0100, Peter Zijlstra wrote: > > On Sun, Nov 11, 2018 at 11:43:52AM -0800, Paul E. McKenney wrote: > > > Now that synchronize_rcu() waits for preempt-disable regions of code > > > as well as RCU read-side critical sections, synchronize_sched() can be > > > replaced by synchronize_rcu(). This commit therefore makes this change. > > > > Yes, but it also waits for an actual RCU quiestent state, which makes > > synchoinize_rcu() potentially much more expensive than an actual > > synchronize_sched(). > > None of the readers have changed. > > For the updaters, if CONFIG_PREEMPT=n, synchronize_rcu() and > synchronize_sched() always were one and the same. When CONFIG_PREEMPT=y, > synchronize_rcu() and synchronize_sched() are now one and the same.
The Changelog does not state this; and does the commit that makes that happen state the regression potential? > > So why are we doing this? > > Given that synchronize_rcu() and synchronize_sched() are now always one > and the same, this is a distinction without a difference. The Changelog did not state a reason for the patch. Therefore it is a bad patch.