Posix timers vs exec problems

[Oleg Nesterov <oleg@tv-sign.ru>]

Roland, George, I think posix timers have problems with de_thread().

First, when non-leader thread does exec it calls release_task(->group_leader)
before calling exit_itimers(). This means that send_group_sigqueue() can oops
after taking tasklist_lock while doing spin_lock_irqsave(->sighand->siglock).
This is easy to fix.

cpu-timers have another problem. In !CPUCLOCK_PERTHREAD() case we attach the
timer to ->signal->cpu_timers, so these timers (when created by another process)
will survive after exec. However they will continue to profile execed process
only if it was group_leader who did exec, otherwise posix_cpu_timer_schedule()
will notice that timer->it.cpu.task has ->signal == NULL and stop this timer.

So, should de_thread() call posix_cpu_timers_exit_group() after exit_itimers()
thus stoping cpu-timers after exec? Another option is to change ->it.cpu.task for
each timer in ->signal->cpu_timers, this way cpu-timers will continue to work.
But this is not trivial.

Oleg.
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