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From: Oleg Nesterov on 29 Mar 2010 14:20
- With the recent changes 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().

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

This change itself doesn't add the functional changes, currently
it is always called under tasklist/siglock or when we know that
this thread group is already dead (wait, coredump).

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
---

kernel/posix-cpu-timers.c | 21 +++++++--------------
1 file changed, 7 insertions(+), 14 deletions(-)

--- 34-rc1/kernel/posix-cpu-timers.c~cpuacct_1_thread_group_cputime_cleanup_rcu 2010-03-29 18:08:14.000000000 +0200
+++ 34-rc1/kernel/posix-cpu-timers.c 2010-03-29 18:09:15.000000000 +0200
@@ -233,31 +233,24 @@ static int cpu_clock_sample(const clocki

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();
}

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Prev: [PATCH] dma-mapping: Remove WARN_ON in dma_free_coherent
Next: [PATCH -mm 0/4] cputimers/proc: do_task_stat: don't walk through the thread list under ->siglock