md: remove need for mddev_lock() in md_seq_show()

The only access in md_seq_show that could suffer from races
not protected by ->lock is walking the rdev list.
This can receive sufficient protection from 'rcu'.

So use rdev_for_each_rcu() and get rid of mddev_lock().

Now reading /proc/mdstat will never block in md_seq_show.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 3 insertions, 5 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 4db4e4146a35..eb0c3b576d7d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -6950,9 +6950,6 @@ static int md_seq_show(struct seq_file *seq, void *v)
                 return 0;
         }
 
-        if (mddev_lock(mddev) < 0)
-                return -EINTR;
-
         spin_lock(&mddev->lock);
         if (mddev->pers || mddev->raid_disks || !list_empty(&mddev->disks)) {
                 seq_printf(seq, "%s : %sactive", mdname(mddev),
@@ -6966,7 +6963,8 @@ static int md_seq_show(struct seq_file *seq, void *v)
                 }
 
                 sectors = 0;
-                rdev_for_each(rdev, mddev) {
+                rcu_read_lock();
+                rdev_for_each_rcu(rdev, mddev) {
                         char b[BDEVNAME_SIZE];
                         seq_printf(seq, " %s[%d]",
                                 bdevname(rdev->bdev,b), rdev->desc_nr);
@@ -6982,6 +6980,7 @@ static int md_seq_show(struct seq_file *seq, void *v)
                                 seq_printf(seq, "(R)");
                         sectors += rdev->sectors;
                 }
+                rcu_read_unlock();
 
                 if (!list_empty(&mddev->disks)) {
                         if (mddev->pers)
@@ -7025,7 +7024,6 @@ static int md_seq_show(struct seq_file *seq, void *v)
                 seq_printf(seq, "\n");
         }
         spin_unlock(&mddev->lock);
-        mddev_unlock(mddev);
 
         return 0;
 }