Re: [Xenomai-help] kernel oopses when killing realtime task

[Philippe Gerum <rpm@xenomai.org>]

On Fri, 2010-10-08 at 09:01 +0200, Pavel Machek wrote:
> Hi!
> 
> > > I have... quite an interesting setup here.
> > > 
> > > SMP machine, with special PCI card; that card has GPIOs and serial
> > > ports. Unfortunately, there's only one interrupt, shared between
> > > serials and GPIO pins, and serials are way too complex to be handled
> > > by realtime layer.
> > > 
> > > So I ended up with
> > > 
> > >         // we also have an interrupt handler:                                                                                                                 
> > >         ret = rtdm_irq_request(&my_context->irq_handle,
> > >         gpio_rt_config.irq, demo_interrupt,
> > >                                RTDM_IRQTYPE_SHARED,
> > >         context->device->proc_name, my_context);
> > > 
> > > and 
> > > 
> > > static int demo_interrupt(rtdm_irq_t *irq_context)
> > > {
> > >         struct demodrv_context *ctx;
> > >         int           dev_id;
> > >         int           ret = RTDM_IRQ_HANDLED; // usual return value                                                                                           
> > >         unsigned pending, output;
> > > 
> > >         ctx = rtdm_irq_get_arg(irq_context, struct demodrv_context);
> > >         dev_id    = ctx->dev_id;
> > > 
> > >         if (!ctx->ready) {
> > >                 printk(KERN_CRIT "Unexpected interrupt\n");
> > >                 return XN_ISR_PROPAGATE;
> > 
> > Who sets ready and when? Looks racy.
> 
> Debugging aid; yes, this one is racy.
> 
> > >         rtdm_lock_put(&ctx->lock);
> > >  
> > >         /* We need to propagate the interrupt, so that PMC-6L serials                                                                                         
> > >            work. Result is that interrupt latencies can't be                                                                                                  
> > >            guaranteed when serials are in use.  */
> > > 
> > >          return RTDM_IRQ_HANDLED;
> > > }
> > > 
> > > Unregistration is:
> > >         my_context->ready = 0;
> > >         rtdm_irq_disable(&my_context->irq_handle);
> > 
> > Where is rtdm_irq_free? Again, this ready flag looks racy.
> 
> Aha, sorry, I quoted wrong snippet. rtdm_irq_free() follows
> immediately, like this:
> 
> int demo_close_rt(struct rtdm_dev_context   *context,
>                   rtdm_user_info_t          *user_info)
> {
>         struct demodrv_context  *my_context;
>         rtdm_lockctx_t          lock_ctx;
>         // get the context                                                                                                                                    
>         my_context = (struct demodrv_context *)context->dev_private;
> 
>         // if we need to do some stuff with preemption disabled:                                                                                              
>         rtdm_lock_get_irqsave(&my_context->lock, lock_ctx);
> 
>         my_context->ready = 0;
>         rtdm_irq_disable(&my_context->irq_handle);
> 
> 
>         // free irq in RTDM                                                                                                                                   
>         rtdm_irq_free(&my_context->irq_handle);
> 
>         // destroy our interrupt signal/event                                                                                                                 
>         rtdm_event_destroy(&my_context->irq_event);
> 
>         // other stuff here                                                                                                                                   
>         rtdm_lock_put_irqrestore(&my_context->lock, lock_ctx);
> 
>         return 0;
> }
> 
> Now... I'm aware that lock_get/put around irq_free should be
> unneccessary, as should be irq_disable and my ->ready flag. Those were
> my attempts to work around the problem. I'll attach the full source at
> the end.
> 
> > > Unfortunately, when the userspace app is ran and killed repeatedly (so
> > > that interrupt is registered/unregistered all the time), I get
> > > oopses in __ipipe_dispatch_wired() -- it seems to call into the NULL
> > > pointer.
> > > 
> > > I decided that "wired" interrupt when the source is shared between
> > > Linux and Xenomai, is wrong thing, so I disable "wired" interrupts
> > > altogether, but that only moved oops to __virq_end. 
> > 
> > This is wrong. The only way to get a determistically shared IRQs across
> > domains is via the wired path, either using the pattern Gilles cited or,
> > in a slight variation, signaling down via a separate rtdm_nrtsig.
> 
> For now, I'm trying to get it not to oops; deterministic latencies are
> the next topic :-(.

The main issue is that we don't lock our IRQ descriptors (the pipeline
ones) when running the handlers, so another CPU clearing them via
ipipe_virtualize_irq() may well sink the boat...

The unwritten rule has always been to assume that drivers would stop
_and_ drain interrupts on all CPUs before unregistering handlers, then
exiting the code. Granted, that's a bit much.

-- 
Philippe.