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. > 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. So we might as well get rid of the _bh and _sched APIs. (See the tail end of current mainline's include/linux/rcupdate.h.) If you are instead asking why the RCU flavors (RCU-bh, RCU-preempt, and RCU-sched) got merged, it was due to a security incident stemming from confusion between two of the flavor, with the resulting bug turning out to be exploitable. Linus therefore requested that I do something to make this not happen again, which I did. Thanx, Paul