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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 untilea6d290c
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. - Sinceea6d290c
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>
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1 changed files with 7 additions and 14 deletions
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@ -232,31 +232,24 @@ static int cpu_clock_sample(const clockid_t which_clock, struct task_struct *p,
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void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
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{
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struct sighand_struct *sighand;
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struct signal_struct *sig;
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struct signal_struct *sig = tsk->signal;
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struct task_struct *t;
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*times = INIT_CPUTIME;
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times->utime = sig->utime;
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times->stime = sig->stime;
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times->sum_exec_runtime = sig->sum_sched_runtime;
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rcu_read_lock();
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sighand = rcu_dereference(tsk->sighand);
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if (!sighand)
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/* make sure we can trust tsk->thread_group list */
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if (!likely(pid_alive(tsk)))
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goto out;
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sig = tsk->signal;
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t = tsk;
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do {
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times->utime = cputime_add(times->utime, t->utime);
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times->stime = cputime_add(times->stime, t->stime);
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times->sum_exec_runtime += t->se.sum_exec_runtime;
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t = next_thread(t);
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} while (t != tsk);
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times->utime = cputime_add(times->utime, sig->utime);
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times->stime = cputime_add(times->stime, sig->stime);
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times->sum_exec_runtime += sig->sum_sched_runtime;
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} while_each_thread(tsk, t);
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out:
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rcu_read_unlock();
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}
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