On 27/11/13 13:49, Michael S. Tsirkin wrote:
On Wed, Nov 27, 2013 at 12:37:02PM +0100, Heinz Graalfs wrote:
On 27/11/13 11:47, Michael S. Tsirkin wrote:
On Wed, Nov 27, 2013 at 11:32:39AM +0100, Heinz Graalfs wrote:
Code is added to avoid calling blk_cleanup_queue() when the surprize_removal
flag is set due to a disappeared device. It avoid hangs due to incomplete
requests (e.g. in-flight requests). Such requests must be considered as lost.
Ugh. Can't we complete these immediately using detach_unused_buf? If not why?
OK, I will try
I tried virtqueue_detach_unused_buf(). It doesn't seem to solve the
problem. Would that affect block layer in-flight requests anyway? The
function comment also says it should not be used on an active queue.
Isn't there a mechanism to end vring requests for which a
vring_interrupt() is missing? (simulate virtblk_done() with an error)?
At least that's it what would help, I suppose.
If the current remove callback was triggered due to an unregister driver,
and the surprize_removal is not already set (although the actual device
is already gone, e.g. virsh detach), blk_cleanup_queue() would be triggered
resulting in a possible hang. This hang is caused by e.g. 'in-flight' requests
that will never complete. This is a weird situation, and most likely not
'serializable'.
Hmm interesting. Implement some timeout and probe device to make sure
it's still alive?
This patch doesn't try to solve any weird races.
It avoids triggering the block queue cleanup, with potential for a hang,
IFF a device is gone.
but there is always some race, isn't it?
To clarify, why this might not be very elegant, a timer-based
solution for surprise removal during driver cleanup
might be easier than trying to build robust interfaces
to address this esoteric case.
But what worries me is that it's not clear to me that ccw won't
invoke notify in parallel with remove callback.
If this happens there will be use after free.
OK, I agree, calling remove twice or working on freed stuff must not happen.
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