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>

1 files changed, 7 insertions, 14 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 9829646d399c..bf2a6502860a 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();
 }