md/raid10: Remove unnecessary rcu_dereference in raid10_handle_discard

We are seeing the following warning in raid10_handle_discard.
[  695.110751] =============================
[  695.131439] WARNING: suspicious RCU usage
[  695.151389] 4.18.0-319.el8.x86_64+debug #1 Not tainted
[  695.174413] -----------------------------
[  695.192603] drivers/md/raid10.c:1776 suspicious
rcu_dereference_check() usage!
[  695.225107] other info that might help us debug this:
[  695.260940] rcu_scheduler_active = 2, debug_locks = 1
[  695.290157] no locks held by mkfs.xfs/10186.

In the first loop of function raid10_handle_discard. It already
determines which disk need to handle discard request and add the
rdev reference count rdev->nr_pending. So the conf->mirrors will
not change until all bios come back from underlayer disks. It
doesn't need to use rcu_dereference to get rdev.

Cc: stable@vger.kernel.org
Fixes: d30588b2731f ('md/raid10: improve raid10 discard request')
Signed-off-by: Xiao Ni <xni@redhat.com>
Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <songliubraving@fb.com>

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 16977e8..d5d9233 100644 (file)
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1712,6 +1712,11 @@ retry_discard:
        } else
                r10_bio->master_bio = (struct bio *)first_r10bio;
 
+       /*
+        * first select target devices under rcu_lock and
+        * inc refcount on their rdev.  Record them by setting
+        * bios[x] to bio
+        */
        rcu_read_lock();
        for (disk = 0; disk < geo->raid_disks; disk++) {
                struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
@@ -1743,9 +1748,6 @@ retry_discard:
        for (disk = 0; disk < geo->raid_disks; disk++) {
                sector_t dev_start, dev_end;
                struct bio *mbio, *rbio = NULL;
-               struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
-               struct md_rdev *rrdev = rcu_dereference(
-                       conf->mirrors[disk].replacement);
 
                /*
                 * Now start to calculate the start and end address for each disk.
@@ -1775,9 +1777,12 @@ retry_discard:
 
                /*
                 * It only handles discard bio which size is >= stripe size, so
-                * dev_end > dev_start all the time
+                * dev_end > dev_start all the time.
+                * It doesn't need to use rcu lock to get rdev here. We already
+                * add rdev->nr_pending in the first loop.
                 */
                if (r10_bio->devs[disk].bio) {
+                       struct md_rdev *rdev = conf->mirrors[disk].rdev;
                        mbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set);
                        mbio->bi_end_io = raid10_end_discard_request;
                        mbio->bi_private = r10_bio;
@@ -1790,6 +1795,7 @@ retry_discard:
                        bio_endio(mbio);
                }
                if (r10_bio->devs[disk].repl_bio) {
+                       struct md_rdev *rrdev = conf->mirrors[disk].replacement;
                        rbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set);
                        rbio->bi_end_io = raid10_end_discard_request;
                        rbio->bi_private = r10_bio;