Messages in this thread | | | Date | Fri, 11 Jun 2010 16:35:26 +0200 | From | Stanislaw Gruszka <> | Subject | Re: [PATCH 4/5] thread_group_cputime: simplify, document the "alive" check |
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On Fri, Jun 11, 2010 at 01:09:56AM +0200, Oleg Nesterov wrote: > 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.
Hmm, I thought we avoided calling thread_group_cputime() from fastpatch_timer_check(), but seems it is still possible when we call run_posix_cpu_timers() on two different cpus simultaneously ...
> - 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().
I'm not sure how important are values of almost dead task, but perhaps would be better to return times form all threads using as base sig->curr_target in loop.
Stanislaw
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