linux-kernel - Re: [PATCH] rcu/torture: Change order of warning and trace dump

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Message-ID: <alpine.DEB.2.21.2204190826500.1508@somnus>
Date:   Tue, 19 Apr 2022 08:28:30 +0200 (CEST)
From:   Anna-Maria Behnsen <anna-maria@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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, 18 Apr 2022, Paul E. McKenney wrote:

> 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!
> 

Hi Paul,

sorry for the late reply - I was busy with other things last week...

This solution is totally fine for me!

Thanks,

	Anna-Maria