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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>
This commit is contained in:
Frederic Weisbecker 2013-10-11 17:41:11 +02:00 committed by syphyr
parent 82ec39fc47
commit e9f47465d0
1 changed files with 13 additions and 3 deletions
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16
kernel/posix-cpu-timers.c
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@ -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)