rqdm: fix stall when ->map_rq returns DM_MAPIO_REQUEUE
This patch fixes the problem that system may stall if target's ->map_rq
returns DM_MAPIO_REQUEUE in map_request().
E.g. stall happens on 1 CPU box when a dm-mpath device with queue_if_no_path
bounces between all-paths-down and paths-up on I/O load.
It is for 2.6.33-rc6, and no dependency on your editing patches.
Please review and apply.
When target's ->map_rq returns DM_MAPIO_REQUEUE, map_request() requeues
the request and returns to dm_request_fn(). Then, dm_request_fn()
doesn't exit the I/O dispatching loop and continues processing
the requeued request again.
This map and requeue loop can be done with interrupt disabled,
so 1 CPU system can be stalled if this situation happens.
For example, commands below can stall my 1 CPU box within 1 minute or so:
# dmsetup table mp
mp: 0 2097152 multipath 1 queue_if_no_path 0 1 1 service-time 0 1 2 8:144 1 1
# while true; do dd if=/dev/mapper/mp of=/dev/null bs=1M count=100; done &
# while true; do
> dmsetup message mp 0 "fail_path 8:144"
> dmsetup suspend --noflush mp
> dmsetup resume mp
> dmsetup message mp 0 "reinstate_path 8:144"
To fix the problem above, this patch changes dm_request_fn() to exit
the I/O dispatching loop once if a request is requeued in map_request().