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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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