Re: [PATCH 0/3] rcu: synchronize_rcu[_expedited]() related fixes

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, Mar 14, 2022 at 03:35:04PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 14, 2022 at 01:26:10PM -0700, Paul E. McKenney wrote:
> > On Mon, Mar 14, 2022 at 02:37:35PM +0100, Frederic Weisbecker wrote:
> > > 
> > > A few fixes especially for expedited GP polling causing a stall on TREE07,
> > > as reported by Paul.
> > > 
> > > We may still want to optimize start_poll_synchronize_rcu_expedited() on
> > > UP-no-preempt but I think Paul may be implying this while doing other
> > > fixes.
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > > 	rcu/dev
> > > 
> > > HEAD: 6e5fd7e614fd5c8f0fffeaa140b7ea697bfeb096
> > > 
> > > Thanks,
> > > 	Frederic
> > 
> > I have pulled these in for review and testing, thank you!!!  I have
> > started a ~90-minute test and will let you know how that goes.
> 
> And TREE05 doesn't like this much.  I get too-short grace periods.
> TREE05 is unusual in being the one with the kernel build with
> CONFIG_PREEMPT_NONE=y but also with CONFIG_PREEMPT_DYNAMIC=n.
> 
> Running tests of the commits individually.

Ouch, please test the following:

---
From f180dd5809d2c3a6343cbd13f244b7b7f110a506 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Tue, 15 Mar 2022 16:33:38 +0100
Subject: [PATCH] rcutorture: Call preempt_schedule() through static call/key

rcutorture sometimes want to trigger a random scheduler preemption call
while simulating a read delay. However a direct call to
preempt_schedule() is not desirable because it bypasses the static
call/key filter used by CONFIG_PREEMPT_DYNAMIC. This breaks the
no-preempt assumption in case the dynamic preemption mode is "none".

For example rcu_blocking_is_gp() is fooled and abbreviates grace periods
when the CPU runs in no-preempt UP mode.

Fix this with making torture_preempt_schedule() to call through
preempt dynamic static call/key.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 include/linux/torture.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/torture.h b/include/linux/torture.h
index 63fa4196e51c..7038104463e4 100644
--- a/include/linux/torture.h
+++ b/include/linux/torture.h
@@ -118,7 +118,7 @@ void _torture_stop_kthread(char *m, struct task_struct **tp);
 	_torture_stop_kthread("Stopping " #n " task", &(tp))
 
 #ifdef CONFIG_PREEMPTION
-#define torture_preempt_schedule() preempt_schedule()
+#define torture_preempt_schedule() __preempt_schedule()
 #else
 #define torture_preempt_schedule()	do { } while (0)
 #endif
-- 
2.25.1