[PATCH] md: fix soft lockup during resync when sync is repeatedly skipped

[Yunye Zhao <yunye.zhao@linux.alibaba.com>]

md_do_sync()'s main loop advances io_sectors only when I/O is actually
issued (skipped == 0).  When sync_request() keeps returning skipped == 1,
io_sectors never increases, the "last_check + window > io_sectors" test
stays true, and every iteration takes the continue branch:

	sectors = mddev->pers->sync_request(mddev, j, max_sectors, &skipped);
	...
	if (!skipped)
		io_sectors += sectors;
	j += sectors;
	...
	if (last_check + window > io_sectors || j == max_sectors)
		continue;

During recovery or resync of a large array with a sparse bitmap, many
regions that need no syncing are skipped:

	raid10_sync_request()
	  md_bitmap_start_sync()	-> must_sync = false (no bitmap page)
	  /* every mirror skipped */
	  biolist == NULL -> *skipped = 1; return max_sync;

j then traverses the whole skipped range while io_sectors stays
unchanged.  On a non-preemptive kernel the resync thread (mdX_resync)
hogs the CPU for a long time and eventually triggers a soft lockup:

  watchdog: BUG: soft lockup - CPU#149 stuck for 313s! [mdX_resync]
   md_bitmap_start_sync+0x6f/0xe0
   raid10_sync_request+0x2c9/0x1530 [raid10]
   md_do_sync+0x810/0x1030
   md_thread+0xa7/0x150

Add cond_resched() to this continue path.

Signed-off-by: Yunye Zhao <yunye.zhao@linux.alibaba.com>
---
 drivers/md/md.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index d1465bcd86c8..e7411b033490 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9881,9 +9881,10 @@ void md_do_sync(struct md_thread *thread)
 			 */
 			md_new_event();
 
-		if (last_check + window > io_sectors || j == max_sectors)
+		if (last_check + window > io_sectors || j == max_sectors) {
+			cond_resched();
 			continue;
-
+		}
 		last_check = io_sectors;
 	repeat:
 		if (time_after_eq(jiffies, mark[last_mark] + SYNC_MARK_STEP )) {
-- 
2.19.1.6.gb485710b