linux-kernel - Re: try_to_freeze() called with IRQs disabled on ARM

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Date:	Fri, 16 Sep 2011 12:31:40 +0200
From:	Martin Schwidefsky <martin.schwidefsky@...ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linaro-toolchain@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	Ulrich Weigand <Ulrich.Weigand@...ibm.com>
Subject: Re: try_to_freeze() called with IRQs disabled on ARM

Hi Russell,

Russell King - ARM Linux <linux@....linux.org.uk> wrote on 09/02/2011
07:48:12 PM:

> On Fri, Sep 02, 2011 at 07:40:34PM +0200, Ulrich Weigand wrote:
> > Russell King - ARM Linux <linux@....linux.org.uk> wrote on 09/02/2011
> > 07:22:59 PM:
> > > On Fri, Sep 02, 2011 at 04:47:35PM +0200, Ulrich Weigand wrote:
> > > > Assume the scenario you initally describe, where a first signal is
> > > > ignored and leads to system call restart.  With your latest patch,
> > > > you call into syscall_restart which sets everything up to restart
> > > > the call (with interrupts disabled).
> > >
> > > I don't think SIG_IGN signals even set the TIF work flag, so they
> > > never even cause a call into do_signal().  Therefore, as far as
> > > syscalls go, attempting to send a process (eg) a SIGINT which its
> > > handler is set to SIG_IGN results in the process not even being
> > > notified about the attempt - we won't even wake up while the
> > > syscall is sleeping.
> >
> > I don't see why SIG_IGN signals shouldn't set the TIF work flag;
> > the decision whether to ignore a signal is only made once we've
> > got to get_signal_to_deliver.
>
> Yes, having looked deeper, you seem to be right - but only if the thread
> is being ptraced.  If it's not being ptraced, ignored signals don't
> make it that far.
>
> And yes, we can end up processing the interrupt before the SVC is
> executed, which is still a hole.  So we need to avoid doing the
> restart in userspace - which might actually make things easier.
> I'll take a look into that over the weekend.

After some more discussions with Uli I've now created a patch that
hopefully will address the issue on s390. The new code always uses
the TIF_RESTART_SVC work flag to restart the system call if there
is no signal to deliver. Only if there is a signal to deliver the
traditional rewind of the instruction pointer is used to restart
the system call (for -ERESTARTNOINTR and -ERESTARTSYS with a
SA_RESTART signal handler).

In case you are interested in the code is available at
git://git390.marist.edu/pub/scm/linux-2.6.git features
in particular git commit 4a6a001c39ac196530d8378408b61d6d2fee70d9.

blue skies,
   Martin

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