From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH 2/3] ftrace: use struct pid Date: Thu, 04 Dec 2008 05:40:00 -0800 Message-ID: References: <20081204052638.425740534@goodmis.org> <20081204052735.175697908@goodmis.org> <1228395378.13111.23.camel@nimitz> <1228396022.13111.27.camel@nimitz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: In-Reply-To: <1228396022.13111.27.camel@nimitz> (Dave Hansen's message of "Thu, 04 Dec 2008 05:07:02 -0800") Sender: linux-kernel-owner@vger.kernel.org To: Dave Hansen Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Steven Rostedt , Steven Rostedt , containers@lists.osdl.org, Ingo Molnar , Sukadev Bhattiprolu , Andrew Morton List-Id: containers.vger.kernel.org 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. But if you look at find_vpid we should be holding just the rcu lock there. Eric