Starting with the commit 63a225c9fd20, device mapper has an optimization that it will take cheaper table lock (dm_get_live_table_fast instead of dm_get_live_table) if the bio has REQ_NOWAIT. The bios with REQ_NOWAIT must not block in the target request routine, if they did, we would be blocking while holding rcu_read_lock, which is prohibited.
The targets that are suitable for REQ_NOWAIT optimization (and that don't block in the map routine) have the flag DM_TARGET_NOWAIT set. Device mapper will test if all the targets and all the devices in a table support nowait (see the function dm_table_supports_nowait) and it will set or clear the QUEUE_FLAG_NOWAIT flag on it's request queue according to this check. There's a test in submit_bio_noacct: "if ((bio->bi_opf & REQ_NOWAIT) && !blk_queue_nowait(q)) goto not_supported" - this will make sure that REQ_NOWAIT bios can't enter a request queue that doesn't support them. This mechanism works to prevent REQ_NOWAIT bios from reaching dm targets that don't support the REQ_NOWAIT flag (and that may block in the map routine) - except that there is a small race condition: submit_bio_noacct checks if the queue has the QUEUE_FLAG_NOWAIT without holding any locks. Immediatelly after this check, the device mapper table may be reloaded with a table that doesn't support REQ_NOWAIT (for example, if we start moving the logical volume or if we activate a snapshot). However the REQ_NOWAIT bio that already passed the check in submit_bio_noacct would be sent to device mapper, where it could be redirected to a dm target that doesn't support REQ_NOWAIT - the result is sleeping while we hold rcu_read_lock. In order to fix this race, we double-check if the target supports REQ_NOWAIT while we hold the table lock (so that the table can't change under us). Signed-off-by: Mikulas Patocka <[email protected]> Fixes: 63a225c9fd20 ("dm: introduce dm_{get,put}_live_table_bio called from dm_submit_bio") --- drivers/md/dm.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) Index: linux-2.6/drivers/md/dm.c =================================================================== --- linux-2.6.orig/drivers/md/dm.c 2022-06-16 09:20:45.000000000 +0200 +++ linux-2.6/drivers/md/dm.c 2022-06-16 19:39:04.000000000 +0200 @@ -1609,7 +1609,12 @@ static blk_status_t __split_and_process_ ti = dm_table_find_target(ci->map, ci->sector); if (unlikely(!ti)) return BLK_STS_IOERR; - else if (unlikely(ci->is_abnormal_io)) + + if (unlikely((ci->bio->bi_opf & REQ_NOWAIT) != 0) && + unlikely(!dm_target_supports_nowait(ti->type))) + return BLK_STS_NOTSUPP; + + if (unlikely(ci->is_abnormal_io)) return __process_abnormal_io(ci, ti); /* -- dm-devel mailing list [email protected] https://listman.redhat.com/mailman/listinfo/dm-devel
