linux-kernel - Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize

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Date:   Sun, 11 Nov 2018 16:45:28 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched()
 with synchronize_rcu()

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