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Commit bfac7009 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar
Browse files

sched: thread_group_cputime: Simplify, document the "alive" check 



thread_group_cputime() looks as if it is rcu-safe, but in fact this
was wrong until ea6d290c which pins task->signal to task_struct.
It checks ->sighand != NULL under rcu, but this can't help if ->signal
can go away. Fortunately the caller either holds ->siglock, or it is
fastpath_timer_check() which uses current and checks exit_state == 0.

- Since ea6d290c commit tsk->signal is stable, we can read it first
  and avoid the initialization from INIT_CPUTIME.

- Even if tsk->signal is always valid, we still have to check it
  is safe to use next_thread() under rcu_read_lock(). Currently
  the code checks ->sighand != NULL, change it to use pid_alive()
  which is commonly used to ensure the task wasn't unhashed before
  we take rcu_read_lock().

  Add the comment to explain this check.

- Change the main loop to use the while_each_thread() helper.

Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100610230956.GA25921@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 48286d50

kernel/posix-cpu-timers.c

+7 −14
Original line number Original line Diff line number Diff line
@@ -232,31 +232,24 @@ static int cpu_clock_sample(const clockid_t which_clock, struct task_struct *p, 




void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)

void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)

{

{

	struct sighand_struct *sighand;

	struct signal_struct *sig = tsk->signal;

	struct signal_struct *sig;

	struct task_struct *t;

	struct task_struct *t;





	*times = INIT_CPUTIME;

	times->utime = sig->utime;

	times->stime = sig->stime;

	times->sum_exec_runtime = sig->sum_sched_runtime;





	rcu_read_lock();

	rcu_read_lock();

	sighand = rcu_dereference(tsk->sighand);

	/* make sure we can trust tsk->thread_group list */

	if (!sighand)

	if (!likely(pid_alive(tsk)))

		goto out;

		goto out;





	sig = tsk->signal;



	t = tsk;

	t = tsk;

	do {

	do {

		times->utime = cputime_add(times->utime, t->utime);

		times->utime = cputime_add(times->utime, t->utime);

		times->stime = cputime_add(times->stime, t->stime);

		times->stime = cputime_add(times->stime, t->stime);

		times->sum_exec_runtime += t->se.sum_exec_runtime;

		times->sum_exec_runtime += t->se.sum_exec_runtime;



	} while_each_thread(tsk, t);

		t = next_thread(t);

	} while (t != tsk);



	times->utime = cputime_add(times->utime, sig->utime);

	times->stime = cputime_add(times->stime, sig->stime);

	times->sum_exec_runtime += sig->sum_sched_runtime;

out:

out:

	rcu_read_unlock();

	rcu_read_unlock();

}

}