posix-timers: Use sighand lock instead of tasklist_lock for task clock sample

There is no need for the tasklist_lock just to take a process
wide clock sample.

All we need is to get a coherent sample that doesn't race with
exit() and exec():

* exit() may be concurrently reaping a task and flushing its time

* sighand is unstable under exit() and exec(), and the latter also
  result in group leader that can change

To protect against these, locking the target's sighand is enough.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 03c5d6c3e614df09972b767d55973aa16b905077..71a07699a36bdeec2ffd04ac64656a1328f5dad2 100644 (file)
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -271,12 +271,22 @@ static int posix_cpu_clock_get_task(struct task_struct *tsk,
                if (same_thread_group(tsk, current))
                        err = cpu_clock_sample(which_clock, tsk, &rtn);
        } else {
-               read_lock(&tasklist_lock);
+               unsigned long flags;
+               struct sighand_struct *sighand;
 
-               if (tsk->sighand && (tsk == current || thread_group_leader(tsk)))
+               /*
+                * while_each_thread() is not yet entirely RCU safe,
+                * keep locking the group while sampling process
+                * clock for now.
+                */
+               sighand = lock_task_sighand(tsk, &flags);
+               if (!sighand)
+                       return err;
+
+               if (tsk == current || thread_group_leader(tsk))
                        err = cpu_clock_sample_group(which_clock, tsk, &rtn);
 
-               read_unlock(&tasklist_lock);
+               unlock_task_sighand(tsk, &flags);
        }
 
        if (!err)