[PATCH v2 2/3] md: add cond_resched() to md_do_sync()'s skip path

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

When sync_request() reports a skipped region (*skipped == 1),
md_do_sync()'s main loop advances the cursor and takes an early
continue:

	j += sectors;
	...
	if (last_check + window > io_sectors || j == max_sectors)
		continue;

If the personality returns a small span per call (raid10 recovery
returns only 128 sectors), syncing a large, mostly clean array iterates
this branch an enormous number of times without ever yielding the CPU.
On a non-preemptive kernel the resync thread then trips the soft-lockup
watchdog:

  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 a cond_resched().  This does not reduce the wasted iterations; the
excessive iteration count is a raid10 problem addressed separately.

Cc: stable@vger.kernel.org
Signed-off-by: Yunye Zhao <yunye.zhao@linux.alibaba.com>
---
 drivers/md/md.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index d1465bcd86c8..c2e8b203c4db 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9881,8 +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:
-- 
2.19.1.6.gb485710b