Re: [PATCH] Re: [BUG] Race condition with it_real_fn in kernel/itimer.c

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

Steven Rostedt wrote:
>
> So, timer_pending tests if timer->base is NULL, but here we see that
> timer->base IS NULL before the function is called, and as I have said
> earlier, the it_real_arm can be called on two CPUS simultaneously. So
> here's another patch that should fix this race condition too.
>
> [...]
>
> +		/*
> +		 * Call del_timer_sync unconditionally, since we don't
> +		 * know if it is running or not. We also need to unlock
> +		 * the siglock so that the it_real_fn called by ksoftirqd
> +		 * doesn't wait for us.
> +		 */
> +		spin_unlock(&tsk->sighand->siglock);
> +		del_timer_sync(&tsk->signal->real_timer);
> +		spin_lock(&tsk->sighand->siglock);

I don't think this is 100% correct. After del_timer_sync() returns another
thread can come and call do_setitimer() and re-arm the timer (because with
your patch we are dropping tsk->sighand->siglock here). So this patch does
not garantees that the timer is not queued/running after del_timer_sync(),
and the it_real_arm can be called on two CPUS simultaneously again.

There is a try_to_del_timer_sync() in the -mm tree which is suitable here:

	again:
		spin_lock_irq(&tsk->sighand->siglock);
		if (try_to_del_timer_sync(&tsk->signal->real_timer) < 0) {
			spin_unlock_irq(&tsk->sighand->siglock);
			goto again;
		}

Oleg.