Messages in this thread | | | Date | Tue, 3 Feb 2009 19:22:40 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC] process wide itimer cruft |
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On 02/03, Peter Zijlstra wrote: > > ->live -- the number of associated tasks, > ->count -- not quite a refcount?
No, ->count is not a refcount.
Basically, ->count means how many threads didn't pass release_task() yet. Well, actually __exit_signal(), but this doesn't matter. The thread becomes "really dead" after that. Until then, it is still visible to, say, find_task_by_vpid, signals, etc.
But if we have a zombie group leader, it may stay zombie "forever", and ->count doesn't go to zero. So we also have signal->live, when it is zero we know that all sub-threads at least entered do_exit(). For example, we can safely do exit_itimers() when ->live == 0, no other thread can do sys_timer_create() (or any syscall of course).
> Could you shed a bit of light on the distinction between sighand and > signal?
->signal is protected by ->sighand->siglock, and they both cleared "atomically" under ->siglock in __exit_signal. I guess, the only reason for 2 structures is CLONE_SIGHAND which can be used without CLONE_THREAD.
Now, let's look at arch/ia64/kernel/ptrace.c:ptrace_attach_sync_user_rbs()
read_lock(&tasklist_lock); if (child->signal) { ... this task is alive, we can proceed ...
This is correct, but if we want to make ->signal refcountable, we should turn the above check into
if (child->sighand) {
This is the same, but allows use to never clear task->signal.
I'll try to send the patch which does this today, we should also change posix-cpu-timers.c and thats all, if my grepping was right.
> > I think we really need another counter, at least for now. > > Don't rush on my account, Ingo's proposed solution doesn't need this.
OK.
Oleg.
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