From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Hansen Subject: Re: [PATCH 2/3] ftrace: use struct pid Date: Thu, 04 Dec 2008 07:12:27 -0800 Message-ID: <1228403547.13111.73.camel@nimitz> 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 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: containers.vger.kernel.org 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(). You mean because the current task can't go through a quiescent period until it hits userspace, and we can't go to userspace while holding read_lock()? Nah, that's not subtle. ;) > 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. Yup, I see it there. So, any reason not to do this? Brown-bag compile tested. Signed-off-by: Dave Hansen --- linux-2.6.git-dave/include/linux/pid.h | 2 ++ 1 file changed, 2 insertions(+) diff -puN include/linux/pid.h~put-rcu-ops-in-do_each_pid_task include/linux/pid.h --- linux-2.6.git/include/linux/pid.h~put-rcu-ops-in-do_each_pid_task 2008-12-04 06:03:09.000000000 -0800 +++ linux-2.6.git-dave/include/linux/pid.h 2008-12-04 06:19:35.000000000 -0800 @@ -147,6 +147,7 @@ pid_t pid_vnr(struct pid *pid); #define do_each_pid_task(pid, type, task) \ do { \ struct hlist_node *pos___; \ + rcu_read_lock(); \ if (pid != NULL) \ hlist_for_each_entry_rcu((task), pos___, \ &pid->tasks[type], pids[type].node) { @@ -159,6 +160,7 @@ pid_t pid_vnr(struct pid *pid); if (type == PIDTYPE_PID) \ break; \ } \ + rcu_read_unlock(); \ } while (0) #define do_each_pid_thread(pid, type, task) \ _ -- Dave