linux-kernel - Re: [RESEND] [RFC][PATCH X86_32 1/2]: Call do_notify

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Date:	Wed, 26 Oct 2011 20:58:38 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Russell King <rmk@....linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Shuah Khan <shuah.khan@...com>, Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND] [RFC][PATCH X86_32 1/2]: Call do_notify_resume() with
	interrupts enabled

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?
	 http://marc.info/?l=linux-kernel&m=125384722012869

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

Or I misunderstood?

Oleg.

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