Commit 50875788 authored Oct 11, 2013 by Frederic Weisbecker

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>

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)

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