Re: [PATCH 1/3] xen/evtchn: avoid a deadlock when unbinding an event channel

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Jul 19, 2013 at 03:51:58PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.com>
> 
> Unbinding an event channel (either with the ioctl or when the evtchn
> device is closed) may deadlock because disable_irq() is called with
> port_user_lock held which is also locked by the interrupt handler.

So what you are saying is that if the ioctl IOCTL_EVTCHN_UNBIND
is called (and takes an spinlock) and the evtchn_interrupt triggers
it would deadlock?

But isn't this the IRQ variant of spinlock? Which disables interrupts?
Could you perhaps write this out a bit with CPU1 and CPU2 in seperate
columns? I think I must missing something.

Thanks!
> 
> Using get_port_user() to check if a port's user is safe without the
> spin lock (as it's protected by u->bind_mutex in the ioctl) so just
> remove the unnesssary locking.

What about in the interrupt handler? It does not use the mutex?
How will it protect the get_port_user() from being stale?