Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu()

Paul E. McKenney
2018-11-12T00:45:38Z

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