Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758992AbYLDPnx (ORCPT ); Thu, 4 Dec 2008 10:43:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755758AbYLDPlj (ORCPT ); Thu, 4 Dec 2008 10:41:39 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:48786 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757241AbYLDPli (ORCPT ); Thu, 4 Dec 2008 10:41:38 -0500 Subject: Re: [PATCH 2/3] ftrace: use struct pid From: Dave Hansen To: "Eric W. Biederman" Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Steven Rostedt , Steven Rostedt , containers@lists.osdl.org, Ingo Molnar , Sukadev Bhattiprolu , Andrew Morton , Dipankar Sarma In-Reply-To: References: <20081204052638.425740534@goodmis.org> <20081204052735.175697908@goodmis.org> <1228395378.13111.23.camel@nimitz> <1228396022.13111.27.camel@nimitz> Content-Type: text/plain Date: Thu, 04 Dec 2008 07:41:31 -0800 Message-Id: <1228405291.13111.86.camel@nimitz> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1745 Lines: 48 On Thu, 2008-12-04 at 05:40 -0800, Eric W. Biederman wrote: > Dave Hansen writes: > > On Thu, 2008-12-04 at 04:56 -0800, Dave Hansen wrote: > >> On Thu, 2008-12-04 at 04:42 -0800, Eric W. Biederman wrote: > >> > > >> > > +static void clear_ftrace_pid_task(struct pid **pid) > >> > > +{ > >> > > + struct task_struct *p; > >> > > + > >> > rcu_read_lock(); > >> > > >> > > + do_each_pid_task(*pid, PIDTYPE_PID, p) { > >> > > + clear_tsk_trace_trace(p); > >> > > + } while_each_pid_task(*pid, PIDTYPE_PID, p); > >> > rcu_read_unlock() > >> > > >> > > + put_pid(*pid); > >> > > + > >> > > + *pid = NULL; > >> > > +} > >> > >> Could we get away with sticking the rcu_read_{un}lock() inside > those > >> macros? Those are going to get used in pretty high level code and > we're > >> allowed to nest rcu_read_lock(). No danger of deadlocks or lock > >> inversions. > > > > Why don't any of the other users of do_each_pid_task() use > > rcu_read_lock()? They all seem to be under > read_lock(&tasklist_lock) > > (except one is under a write lock of the same). > > We probably should. Historically read_lock(&tasklist_lock) implies > rcu_read_lock(). And the tasklist lock is what we hold when it is > safe. So, Dipankar tells me that you really do need rcu_read_lock/unlock() for the guarantee here; the tasklist lock is not sufficient. The realtime kernel will preempt even those sections covered by spinlocks. -- Dave -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/