Re: [Xenomai-core] arm: Unprotected access to irq_desc field?

[Philippe Gerum <rpm@xenomai.org>]

On Fri, 2010-10-29 at 11:05 +0200, Jan Kiszka wrote:
> Am 29.10.2010 10:27, Philippe Gerum wrote:
> > On Fri, 2010-10-29 at 09:00 +0200, Jan Kiszka wrote:
> >> Am 28.10.2010 21:34, Philippe Gerum wrote:
> >>> On Thu, 2010-10-28 at 21:15 +0200, Gilles Chanteperdrix wrote:
> >>>> Jan Kiszka wrote:
> >>>>> Gilles,
> >>>>>
> >>>>> I happened to come across rthal_mark_irq_disabled/enabled on arm. On
> >>>>> first glance, it looks like these helpers manipulate irq_desc::status
> >>>>> non-atomically, i.e. without holding irq_desc::lock. Isn't this fragile?
> >>>>
> >>>> I have no idea. How do the other architectures do? As far as I know,
> >>>> this code has been copied from there.
> >>>
> >>> Other archs do the same, simply because once an irq is managed by the
> >>> hal, it may not be shared in any way with the regular kernel. So locking
> >>> is pointless.
> >>
> >> Indeed, I missed that all the other archs have this uninlined in hal.c.
> >>
> >> However, this leaves at least a race between xnintr_disable/enable and
> >> XN_ISR_PROPAGATE (ie. the related Linux path) behind.
> > 
> > I can't see why XN_ISR_PROPAGATE would be involved here. This service
> > pends an interrupt in the pipeline log.
> 
> And this finally lets Linux code run that fiddles with irq_desc::status
> as well - potentially in parallel to an unsyncrhonized
> xnintr_irq_disable in a different context. That's the problem.

Propagation happens in primary domain. When is this supposed to conflict
on the same CPU with linux?

> 
> > 
> >>  Not sure if it
> >> matters practically - but risking silent breakage for this micro
> >> optimization?
> > 
> > It was not meant as an optimization; we may not grab the linux
> > descriptor lock in this context because we may enter it in primary mode.
> 
> Oh, that lock isn't harden as I somehow assumed. This of course
> complicates things.
> 
> > 
> >>  Is disabling/enabling really in the highly
> >> latency-critical anywhere? Otherwise, I would suggest to just plug this
> >> by adding the intended lock for this field.
> > 
> > The caller is expected to manage locking; AFAICS the only one who does
> > not is the RTAI skin, which is obsolete and removed in 2.6.x, so no big
> > deal.
> 
> The problem is that IRQ forwarding to Linux may let this manipulation
> race with plain Linux code, thus has to synchronize with it. It is a
> corner case (no one is supposed to pass IRQs down blindly anyway - if at
> all), but it should at least be documented ("Don't use disable/enable
> together with IRQ forwarding unless you acquire the descriptor lock
> properly!").
> 
> BTW, do we need to track the descriptor state in primary mode at all?
> 

That is the real issue. I don't see the point of doing this with the
current kernel code.

> Jan
> 

-- 
Philippe.