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Subject[tip:sched/core] sched: thread_group_cputime: Simplify, document the "alive" check
Commit-ID:  bfac7009180901f57f20a73c53c3e57b1ce75a1b
Gitweb: http://git.kernel.org/tip/bfac7009180901f57f20a73c53c3e57b1ce75a1b
Author: Oleg Nesterov <oleg@redhat.com>
AuthorDate: Fri, 11 Jun 2010 01:09:56 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 18 Jun 2010 10:46:56 +0200

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: 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>
---
kernel/posix-cpu-timers.c | 21 +++++++--------------
1 files changed, 7 insertions(+), 14 deletions(-)
diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9829646..bf2a650 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -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();
}

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