[PATCH 6.4 037/800] md/raid10: fix io loss while replacement replace rdev

Greg Kroah-Hartman
2023-07-16T19:54:32Z

From: Li Nan <linan122@huawei.com>

[ Upstream commit 2ae6aaf76912bae53c74b191569d2ab484f24bf3 ]

When removing a disk with replacement, the replacement will be used to
replace rdev. During this process, there is a brief window in which both
rdev and replacement are read as NULL in raid10_write_request(). This
will result in io not being submitted but it should be.

//remove //write
raid10_remove_disk raid10_write_request
mirror->rdev = NULL
read rdev -> NULL
mirror->rdev = mirror->replacement
mirror->replacement = NULL
read replacement -> NULL

Fix it by reading replacement first and rdev later, meanwhile, use smp_mb()
to prevent memory reordering.

Fixes: 475b0321a4df ("md/raid10: writes should get directed to replacement as well as original.")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230602091839.743798-3-linan666@huaweicloud.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/raid10.c | 22 ++++++++++++++++++----
1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 0acb4c103c10e..c059533b890f9 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -779,8 +779,16 @@ static struct md_rdev *read_balance(struct r10conf *conf,
disk = r10_bio->devs[slot].devnum;
rdev = rcu_dereference(conf->mirrors[disk].replacement);
if (rdev == NULL || test_bit(Faulty, &rdev->flags) ||
- r10_bio->devs[slot].addr + sectors > rdev->recovery_offset)
+ r10_bio->devs[slot].addr + sectors >
+ rdev->recovery_offset) {
+ /*
+ * Read replacement first to prevent reading both rdev
+ * and replacement as NULL during replacement replace
+ * rdev.
+ */
+ smp_mb();
rdev = rcu_dereference(conf->mirrors[disk].rdev);
+ }
if (rdev == NULL ||
test_bit(Faulty, &rdev->flags))
continue;
@@ -1479,9 +1487,15 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,

for (i = 0; i < conf->copies; i++) {
int d = r10_bio->devs[i].devnum;
- struct md_rdev *rdev = rcu_dereference(conf->mirrors[d].rdev);
- struct md_rdev *rrdev = rcu_dereference(
- conf->mirrors[d].replacement);
+ struct md_rdev *rdev, *rrdev;
+
+ rrdev = rcu_dereference(conf->mirrors[d].replacement);
+ /*
+ * Read replacement first to prevent reading both rdev and
+ * replacement as NULL during replacement replace rdev.
+ */
+ smp_mb();
+ rdev = rcu_dereference(conf->mirrors[d].rdev);
if (rdev == rrdev)
rrdev = NULL;
if (rdev && (test_bit(Faulty, &rdev->flags)))
--
2.39.2