Re: [Xen-devel] [PATCH 2/2] xl: disable events earlier for shutdown event

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Wei Liu writes ("Re: [PATCH 2/2] xl: disable events earlier for shutdown 
event"):
> On Thu, Feb 02, 2017 at 03:52:41PM +0000, Ian Jackson wrote:
> > But I think I don't really understand what the original bug is.
> 
> The original bug is that:

Ah.

> 1. boot a guest with no disks, so diskws is NULL, num_disks == 0
> 2. some removable disks are added, and domain config updated
> 3. guest reboots, xl gets shutdown event
> 4. handle_domain_death gets the latest d_config, num_disks != 0 now
> 5. try to disable disk eject events with evdisable_disk_ejects -> BOOM
> 
> So basically 5 needs to happen before 4. Moving that snippet seems to do
> the trick, and then freeing diskws + setting it to NULL makes the
> reboot path automatically re-register diskws.

Surely the right answer is to make evdisable_disk_ejects tolerate
whatever it finds in diskws (including diskws==NULL).  It could be
idempotent too.

That way nothing depends on the exact ordering.

Ian.

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