• Kiyoshi Ueda's avatar
    dm mpath: fix stall when requeueing io · 9eef87da
    Kiyoshi Ueda authored
    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.
    
    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" \
      > done
    
    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().
    Signed-off-by: default avatarKiyoshi Ueda <k-ueda@ct.jp.nec.com>
    Signed-off-by: default avatarJun'ichi Nomura <j-nomura@ce.jp.nec.com>
    Cc: stable@kernel.org
    Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
    9eef87da
dm.c 61.1 KB