[PATCH -next 1/8] md/raid10: prevent soft lockup while flush writes

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Currently, there is no limit for raid1/raid10 plugged bio. While flushing
writes, raid1 has cond_resched() while raid10 doesn't, and too many
writes can cause soft lockup.

Follow up soft lockup can be triggered easily with writeback test for
raid10 with ramdisks:

watchdog: BUG: soft lockup - CPU#10 stuck for 27s! [md0_raid10:1293]
Call Trace:
 <TASK>
 call_rcu+0x16/0x20
 put_object+0x41/0x80
 __delete_object+0x50/0x90
 delete_object_full+0x2b/0x40
 kmemleak_free+0x46/0xa0
 slab_free_freelist_hook.constprop.0+0xed/0x1a0
 kmem_cache_free+0xfd/0x300
 mempool_free_slab+0x1f/0x30
 mempool_free+0x3a/0x100
 bio_free+0x59/0x80
 bio_put+0xcf/0x2c0
 free_r10bio+0xbf/0xf0
 raid_end_bio_io+0x78/0xb0
 one_write_done+0x8a/0xa0
 raid10_end_write_request+0x1b4/0x430
 bio_endio+0x175/0x320
 brd_submit_bio+0x3b9/0x9b7 [brd]
 __submit_bio+0x69/0xe0
 submit_bio_noacct_nocheck+0x1e6/0x5a0
 submit_bio_noacct+0x38c/0x7e0
 flush_pending_writes+0xf0/0x240
 raid10d+0xac/0x1ed0

This patch fix the problem by adding cond_resched() to raid10 like what
raid1 did.

Note that unlimited plugged bio still need to be optimized because in
the case of writeback lots of dirty pages, this will take lots of memory
and io latecy is quite bad.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/raid10.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 6590aa49598c..a116b7c9d9f3 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -921,6 +921,7 @@ static void flush_pending_writes(struct r10conf *conf)
 			else
 				submit_bio_noacct(bio);
 			bio = next;
+			cond_resched();
 		}
 		blk_finish_plug(&plug);
 	} else
@@ -1140,6 +1141,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
 		else
 			submit_bio_noacct(bio);
 		bio = next;
+		cond_resched();
 	}
 	kfree(plug);
 }
-- 
2.39.2