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Commit 50875788 authored by Frederic Weisbecker's avatar Frederic Weisbecker
Browse files

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: default avatarFrederic 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>
parent 33ab0fec

kernel/posix-cpu-timers.c

+13 −3
Original line number Diff line number Diff line
@@ -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;



		/*

		 * 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->sighand && (tsk == current || thread_group_leader(tsk)))

		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)