Re: [RESEND] [RFC][PATCH X86_32 1/2]: Call do_notify_resume() withinterrupts enabled

From: Oleg Nesterov
Date: Wed Oct 26 2011 - 15:03:46 EST

On 10/26, Russell King wrote:
> I've been toying with a similar patch for ARM, but I keep feeling uneasy
> about having interrupts enabled in this path (even though they get enabled
> in the depths of the signal handling code.)
> I worry about are race condition like the following:
> syscall enter
> ...
> syscall returns -ERESTARTNOHAND
> check for signal
> signal pending, but no handler, setup for restart
> interrupt happens, sets need_resched
> need_resched set
> switch to another thread
> ...
> something happens which queues SIGIO
> switch back to this thread

I don't understand how "interrupts disabled" can help... A signal
can come without preempt_schedule().

> check for signal
> signal pending, has handler, but we've setup for a restart
> return to userspace
> run SIGIO handler
> restart syscall
> This feels like it violates the expectations of the syscall being
> restarted - which explicitly asks to be restarted only if there wasn't
> a handler run.

But this doesn't differ from the case when this signal comes after
the sycall was already restarted?

> However, that doesn't solve the (probably unsolvable) case where an
> ERESTARTSYS syscall is interrupted by a SA_RESTART-marked handler, and
> while that handler is running it is then interrupted by a non-SA_RESTART-
> marked handler. I think that is far too an obscure case to care about
> though.

If I understand correctly, this was already discussed:

Re: HR timers prevent an itimer from generating EINTR?

Unfortunately, doesn't show the authoritative reply from
Roland, but he agreed with "not a problem".

Or I misunderstood?


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