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

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Date:	Wed, 26 Oct 2011 16:10:05 +0100
From:	Russell King <rmk@....linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	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 Wed, Oct 26, 2011 at 02:38:39PM +0200, Linus Torvalds wrote:
> Ingo, Thomas, I think this is your call, but it seems valid,

Hi Linus,

I guess I should've talked to you about this during a moment during the
kernel summit, but as I'm now back home it'll have to be email.

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
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.

I've been working on the assumption that this is a problem and we should
do something about it - but it's non-trivial to solve all the corner cases.
We can do a lot better with the restarting if we delay setting up for a
restart until either we setup the user stack for the sig handler or
immediately before returning to userspace (with a TIF flag.)

If you're interested in seeing where I got to, the patch is available at:
	https://lkml.org/lkml/2011/8/25/231

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.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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