linux-kernel - Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with _

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Message-ID: <20200205132919.GU2935@paulmck-ThinkPad-P72>
Date:   Wed, 5 Feb 2020 05:29:19 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Amol Grover <frextrite@...il.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
Subject: Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu

On Wed, Feb 05, 2020 at 08:19:21AM -0500, Steven Rostedt wrote:
> On Wed, 5 Feb 2020 08:14:09 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Wed, 5 Feb 2020 05:11:10 -0800
> > "Paul E. McKenney" <paulmck@...nel.org> wrote:
> > 
> > > I strongly recommend a comment stating why disabling preemption prevents
> > > ftrace_graph_hash from going away.  I see the synchronize_rcu() after
> > > the rcu_assign_pointer() in ftrace_graph_release(), but I don't see
> > > anything that waits on CPUs that RCU is not watching.
> > > 
> > > Of course, event tracing -makes- RCU watch when needed, but if that
> > > was set up, then lockdep would not have complained.
> > > 
> > > So what am I missing?  
> > 
> > Keep looking in your INBOX and look at the patch I asked you to ack or
> > complain about ;-) 

Not yet seeing it, but...

> Actually, looking at the code myself, it appears to be missing the
> ftrace_sync. Thus, this is a bug, and requires the ftrace sync, as
> synchronize_rcu() is not strong enough here.
> 
> Patch in process!

Thank you for chasing it down!

							Thanx, Paul