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>

1 files changed, 13 insertions, 3 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 03c5d6c3e614..71a07699a36b 100644
--- 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)