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Message-ID: <20220418185042.GA2420391@paulmck-ThinkPad-P17-Gen-1>
Date: Mon, 18 Apr 2022 11:50:42 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Benedikt Spranger <b.spranger@...utronix.de>
Subject: Re: [PATCH] rcu/torture: Change order of warning and trace dump
On Mon, Apr 11, 2022 at 11:09:15AM -0700, Paul E. McKenney wrote:
> On Mon, Apr 11, 2022 at 05:19:03PM +0200, Anna-Maria Behnsen wrote:
> > Dumping a big ftrace buffer could lead to a RCU stall. So there is the
> > ftrace buffer and the stall information which needs to be printed. When
> > there is additionaly a WARN_ON() which describes the reason for the ftrace
> > buffer dump and the WARN_ON() is executed _after_ ftrace buffer dump, the
> > information get lost in the middle of the RCU stall information.
> >
> > Therefore print WARN_ON() message before dumping the ftrace buffer in
> > rcu_torture_writer().
> >
> > Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
> > Reviewed-by: Benedikt Spranger <b.spranger@...utronix.de>
>
> Hello, Anna-Maria!
>
> Good point, but we get caught out either way. Either we take the chance
> of losing the WARN() message as you say, or we take the chance of the
> activity in the WARN() message overwriting needed information in the
> trace buffer.
>
> Would it work to shut off tracing, do the WARN(), and only then do the
> rcu_ftrace_dump()?
For example, as shown the the updated patch below currently queued on -rcu
for further review and testing?
If this is problematic, please let me know!
Thanx, Paul
------------------------------------------------------------------------
commit cdbc93534da1bba3f498783473361b2f26dc65d6
Author: Anna-Maria Behnsen <anna-maria@...utronix.de>
Date: Mon Apr 11 17:19:03 2022 +0200
rcu/torture: Change order of warning and trace dump
Dumping a big ftrace buffer could lead to a RCU stall. So there is the
ftrace buffer and the stall information which needs to be printed. When
there is additionaly a WARN_ON() which describes the reason for the ftrace
buffer dump and the WARN_ON() is executed _after_ ftrace buffer dump, the
information get lost in the middle of the RCU stall information.
Therefore print WARN_ON() message before dumping the ftrace buffer in
rcu_torture_writer().
[ paulmck: Add tracing_off() to avoid cruft from WARN(). ]
Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
Reviewed-by: Benedikt Spranger <b.spranger@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 2b40a8f6d2a0..0885a66f9d76 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1369,8 +1369,9 @@ rcu_torture_writer(void *arg)
if (list_empty(&rcu_tortures[i].rtort_free) &&
rcu_access_pointer(rcu_torture_current) !=
&rcu_tortures[i]) {
- rcu_ftrace_dump(DUMP_ALL);
+ tracing_off();
WARN(1, "%s: rtort_pipe_count: %d\n", __func__, rcu_tortures[i].rtort_pipe_count);
+ rcu_ftrace_dump(DUMP_ALL);
}
if (stutter_waited)
sched_set_normal(current, oldnice);
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