minor GEOM disk API change coming

[Andrey V. Elsukov]

On 21.06.2012 20:48, Kenneth D. Merry wrote:
>>> 	In this case, the GEOM disk class instance has been created by
>>> 	disk_create(), and the taste of the disk is queued in the GEOM
>>> 	event queue.
>>> 	
>>> 	While that event is queued, the da(4) instance goes away.  When the
>>> 	open call comes into the da(4) driver, it dereferences the freed
>>> 	(but non-NULL) peripheral pointer provided by GEOM, which results
>>> 	in a panic.
>>
>> I think this situation is very specific for the GEOM_DISK class, and
>> this callback will be less useful for other classes.
>> Does g_cancel_event() cannot help you prevent tasting?
> 
> Calling g_cancel_event(), for instance from disk_gone(), would not
> completely close the race condition.  It can't cancel an event that is
> already in progress, and it is possible for the peripheral to go away while
> the event is marked in progress but before the taste gets far enough into
> daopen() to acquire a reference to the peripheral.

If i understand correctly your patch, you acquires a reference to the
periph and release it when g_destroy_provider finished. What if you will
queue some custom event from the disk_gone() that will call
cddiskgonecb()? Does it close the race? This event will be executed
after the taste completes.

-- 
WBR, Andrey V. Elsukov

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