How to reissue stuck inflight i/o

[Spelic <spelic@shiftmail.org>]

Hello all,
we just had a case in which some I/Os were apparently stuck inflight in 
an LV aka DM (visible in iostat as nonzero avgqu-sz for an dm-X device) 
for a long time, such as at least 20 minutes, but all layers below it 
had zero inflight I/O (MD RAID and then the disks), so DM and the above 
layers were waiting endlessly.
This was with an old kernel 2.6.24-something .
I wasn't able to debug further. After 30 minutes or so it resolved by 
itself without leaving anything in dmesg or anywhere else.

Is there a way to reissue inflight I/O to lower layers (such as what 
happens transparently with the 5 SCSI retries after SCSI timeout) for 
DM, or at least kill such I/O so that above layers receive an I/O error 
and move on?
I was thinking at some dmsetup command but was not clear to me which.
What about a dmsetup suspend and then resume? I didn't think about 
trying this, at that time.

Thanks
S.