Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section

Joel Fernandes
2019-07-13T03:10:13Z

On Fri, Jul 12, 2019 at 11:01:50PM -0400, Joel Fernandes wrote:
> On Fri, Jul 12, 2019 at 04:32:06PM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 12, 2019 at 05:35:59PM -0400, Joel Fernandes wrote:
> > > On Fri, Jul 12, 2019 at 01:00:18PM -0400, Joel Fernandes (Google) wrote:
> > > > The rcu/sync code was doing its own check whether we are in a reader
> > > > section. With RCU consolidating flavors and the generic helper added in
> > > > this series, this is no longer need. We can just use the generic helper
> > > > and it results in a nice cleanup.
> > > >
> > > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > >
> > > Hi Oleg,
> > > Slightly unrelated to the patch,
> > > I tried hard to understand this comment below in percpu_down_read() but no dice.
> > >
> > > I do understand how rcu sync and percpu rwsem works, however the comment
> > > below didn't make much sense to me. For one, there's no readers_fast anymore
> > > so I did not follow what readers_fast means. Could the comment be updated to
> > > reflect latest changes?
> > > Also could you help understand how is a writer not able to change
> > > sem->state and count the per-cpu read counters at the same time as the
> > > comment tries to say?
> > >
> > > /*
> > > * We are in an RCU-sched read-side critical section, so the writer
> > > * cannot both change sem->state from readers_fast and start checking
> > > * counters while we are here. So if we see !sem->state, we know that
> > > * the writer won't be checking until we're past the preempt_enable()
> > > * and that once the synchronize_rcu() is done, the writer will see
> > > * anything we did within this RCU-sched read-size critical section.
> > > */
> > >
> > > Also,
> > > I guess we could get rid of all of the gp_ops struct stuff now that since all
> > > the callbacks are the same now. I will post that as a follow-up patch to this
> > > series.
> >
> > Hello, Joel,
> >
> > Oleg has a set of patches updating this code that just hit mainline
> > this week. These patches get rid of the code that previously handled
> > RCU's multiple flavors. Or are you looking at current mainline and
> > me just missing your point?
> >
>
> Hi Paul,
> You are right on point. I have a bad habit of not rebasing my trees. In this
> case the feature branch of mine in concern was based on v5.1. Needless to
> say, I need to rebase my tree.
>
> Yes, this sync clean up patch does conflict when I rebase, but other patches
> rebase just fine.
>
> The 2 options I see are:
> 1. Let us drop this patch for now and I resend it later.
> 2. I resend all patches based on Linus's master branch.

Below is the updated patch based on Linus master branch:

---8<-----------------------

From 5f40c9a07fcf3d6dafc2189599d0ba9443097d0f Mon Sep 17 00:00:00 2001
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Date: Fri, 12 Jul 2019 12:13:27 -0400
Subject: [PATCH v2.1 3/9] rcu/sync: Remove custom check for reader-section

The rcu/sync code was doing its own check whether we are in a reader
section. With RCU consolidating flavors and the generic helper added in
this series, this is no longer need. We can just use the generic helper
and it results in a nice cleanup.

Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
include/linux/rcu_sync.h | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
index 9b83865d24f9..0027d4c8087c 100644
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -31,9 +31,7 @@ struct rcu_sync {
*/
static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
{
- RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
- !rcu_read_lock_bh_held() &&
- !rcu_read_lock_sched_held(),
+ RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),
"suspicious rcu_sync_is_idle() usage");
return !READ_ONCE(rsp->gp_state); /* GP_IDLE */
}
--
2.22.0.510.g264f2c817a-goog