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McKenney" To: Steven Rostedt Cc: Joel Fernandes , Amol Grover , Ingo Molnar , linux-kernel@vger.kernel.org, linux-kernel-mentees@lists.linuxfoundation.org, Madhuparna Bhowmik Subject: Re: [PATCH] tracing: Annotate ftrace_graph_hash pointer with __rcu Message-ID: <20200205132919.GU2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200201072703.17330-1-frextrite@gmail.com> <20200203163301.GB85781@google.com> <20200204200116.479f0c60@oasis.local.home> <20200205131110.GT2935@paulmck-ThinkPad-P72> <20200205081409.4550aa67@oasis.local.home> <20200205081921.34b19d9a@oasis.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200205081921.34b19d9a@oasis.local.home> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 05, 2020 at 08:19:21AM -0500, Steven Rostedt wrote: > On Wed, 5 Feb 2020 08:14:09 -0500 > Steven Rostedt wrote: > > > On Wed, 5 Feb 2020 05:11:10 -0800 > > "Paul E. McKenney" 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