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SubjectRe: [RFC,PATCH] Use RCU to protect tasklist for unicast signals
Paul E. McKenney wrote:
>
> I have indeed been thinking along these lines, but all of the devil plans
> that I have come up thus far are quite intrusive, and might have some
> performance problems in some situations. So it seems best to remove
> tasklist_lock in steps:
>
> 1. Single-recipient catch and ignore cases.
>
> 2. Single-recipient stop/continue cases.
>
> 3. Single-recipient fatal cases.
>
> 4. Single-process multi-threaded stop/continue cases.
>
> 5. Single-process multi-threaded fatal cases.
>
> 6. And on to process-group cases.

Paul, I am not an expert at all, but honestly I don't see how
this could be achieved. This lock is heavily overloaded for
quite different purposes. I think that may be it makes sense
to try other steps, for example (random order):

1. Tasklist protects ->sighand changing (de_thread) - rework
sighand access/locking.

2. Tasklist protects reparenting - fix this.

.......
N. PTRACE!!! Well, I close my eyes immediately when I see this
word in the sources.

Only then we can eliminate tasklist locking from signal sending
path. But I don't see the easy way to solve any of these 1 - N
problems.

Currently I don't see how your patch could be "fixed" for SIGCONT
case, except very ugly:

kill_proc_info(sig)
{
p = find_task_by_pid(pid);
if (sig == SIGCONT)
read_lock(&tasklist_lock);
error = group_send_sig_info(...);
...
}
But there are other problems too.

Look at __group_complete_signal(), it scans p->pids[PIDTYPE_TGID].pid_list
list to find a a suitable thread. What if 'p' does clone(CLONE_THREAD) now?
Let's look at copy_process(), it does attach_pid(p, PIDTYPE_TGID, p->tgid)
under the lovely tasklist_lock again.

So, I don't beleive we can solve even the simplest case (single-recipient,
non fatal, non stop/cont) without significant locking rework.

I hope that your patch will stimulate this work.

Oleg.
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