[PATCH v2 3/3] hvc/xen: fix console unplug

Juergen Gross jgross at suse.com
Mon Oct 23 19:14:32 AEDT 2023

On 20.10.23 18:15, David Woodhouse wrote:
> From: David Woodhouse <dwmw at amazon.co.uk>
> On unplug of a Xen console, xencons_disconnect_backend() unconditionally
> calls free_irq() via unbind_from_irqhandler(), causing a warning of
> freeing an already-free IRQ:
> (qemu) device_del con1
> [   32.050919] ------------[ cut here ]------------
> [   32.050942] Trying to free already-free IRQ 33
> [   32.050990] WARNING: CPU: 0 PID: 51 at kernel/irq/manage.c:1895 __free_irq+0x1d4/0x330
> It should be using evtchn_put() to tear down the event channel binding,
> and let the Linux IRQ side of it be handled by notifier_del_irq() through
> the HVC code.
> On which topic... xencons_disconnect_backend() should call hvc_remove()
> *first*, rather than tearing down the event channel and grant mapping
> while they are in use. And then the IRQ is guaranteed to be freed by
> the time it's torn down by evtchn_put().
> Since evtchn_put() also closes the actual event channel, avoid calling
> xenbus_free_evtchn() except in the failure path where the IRQ was not
> successfully set up.
> However, calling hvc_remove() at the start of xencons_disconnect_backend()
> still isn't early enough. An unplug request is indicated by the backend
> setting its state to XenbusStateClosing, which triggers a notification
> to xencons_backend_changed(), which... does nothing except set its own
> frontend state directly to XenbusStateClosed without *actually* tearing
> down the HVC device or, you know, making sure it isn't actively in use.
> So the backend sees the guest frontend set its state to XenbusStateClosed
> and stops servicing the interrupt... and the guest spins for ever in the
> domU_write_console() function waiting for the ring to drain.
> Fix that one by calling hvc_remove() from xencons_backend_changed() before
> signalling to the backend that it's OK to proceed with the removal.
> Tested with 'dd if=/dev/zero of=/dev/hvc1' while telling Qemu to remove
> the console device.
> Signed-off-by: David Woodhouse <dwmw at amazon.co.uk>
> Cc: stable at vger.kernel.org

Reviewed-by: Juergen Gross <jgross at suse.com>


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