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

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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: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100610230956.GA25921@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2010-06-11 01:09:56 +02:00 committed by Ingo Molnar
parent 48286d5088
commit bfac700918
1 changed files with 7 additions and 14 deletions
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21
kernel/posix-cpu-timers.c
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@ -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)
{
struct sighand_struct *sighand;
struct signal_struct *sig;
struct signal_struct *sig = tsk->signal;
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();
sighand = rcu_dereference(tsk->sighand);
if (!sighand)
/* make sure we can trust tsk->thread_group list */
if (!likely(pid_alive(tsk)))
goto out;
sig = tsk->signal;
t = tsk;
do {
times->utime = cputime_add(times->utime, t->utime);
times->stime = cputime_add(times->stime, t->stime);
times->sum_exec_runtime += t->se.sum_exec_runtime;
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;
} while_each_thread(tsk, t);
out:
rcu_read_unlock();
}