On Wed, Aug 13, 2014 at 06:57:18AM -0700, Stephen Boyd wrote:
> Commit 1a6b69b6548c (ARM: gic: add CPU migration support,
> 2012-04-12) introduced an acquisition of the irq_controller_lock
> in gic_raise_softirq() which can lead to a spinlock recursion if
> the gic_arch_extn hooks call into the scheduler (via complete()
> or wake_up(), etc.). This happens because gic_arch_extn hooks are
> normally called with the irq_controller_lock held and calling
> into the scheduler may cause us to call smp_send_reschedule()
> which will grab the irq_controller_lock again. Here's an example
> from a vendor kernel (note that the gic_arch_extn hook code here
> isn't actually in mainline):
Here's a question: why would you want to call into the scheduler from
the gic_arch_extn code?
Oh. My. God. Thomas, what have you done to the generic IRQ layer?
This is /totally/ unsafe:
void disable_irq(unsigned int irq)
{
if (!__disable_irq_nosync(irq))
synchronize_irq(irq);
}
static int __disable_irq_nosync(unsigned int irq)
{
unsigned long flags;
struct irq_desc *desc = irq_get_desc_buslock(irq, &flags, IRQ_GET_DESC_CHECK_GLOBAL);
if (!desc)
return -EINVAL;
__disable_irq(desc, irq, false);
irq_put_desc_busunlock(desc, flags);
return 0;
}
void __disable_irq(struct irq_desc *desc, unsigned int irq, bool suspend)
{
if (suspend) {
if (!desc->action || (desc->action->flags & IRQF_NO_SUSPEND))
return;
desc->istate |= IRQS_SUSPENDED;
}
if (!desc->depth++)
irq_disable(desc);
}
You realise that disable_irq() and enable_irq() can be called by
concurrently by different drivers for the /same/ interrupt. For
starters, that post-increment there is completely unprotected against
races. Secondly, the above is completely racy against a concurrent
enable_irq() - what if we're in disable_irq(), we've incremented
depth, but have yet to call irq_disable(). The count now has a
value of 1.
We then preempt, and run another thread which calls enable_irq()
on it. This results in the depth being decremented, and the IRQ
is now enabled.
We resume the original thread, and continue to call irq_disable(),
resulting in the interrupt being disabled.
That's not nice (the right answer is that it's strictly an unbalanced
enable_irq(), but that's no excuse here.)
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