[PATCH 2/5] fs: make ustat() and quotactl() killable in lookup

[Josef Bacik]

user_get_super() looks up the superblock for a device number on behalf of
ustat(2) and quotactl(2).  If that superblock is still being mounted it
waits for SB_BORN, and then for s_umount, in super_lock().  With a mount
which is stuck on its device neither system call ever returns, and neither
can be killed.

Both fail if there is no superblock for the device, which is what
user_get_super() returning NULL means to them.  Use super_lock_killable()
and return NULL when the task is killed.

Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 fs/super.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index 7164c92fe70e..40f92ffa39c1 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1172,9 +1172,15 @@ struct super_block *user_get_super(dev_t dev, bool excl)
 
 	for (sb_dev = super_dev_first(dev); sb_dev; sb_dev = super_dev_next(sb_dev)) {
 		struct super_block *sb = sb_dev->sd_sb;
+		int ret;
 
-		if (!super_lock(sb, excl))
+		ret = super_lock_killable(sb, excl);
+		if (ret == -ENOENT)
 			continue;
+		if (ret) {
+			super_dev_put(sb_dev);
+			return NULL;
+		}
 
 		/* The pinned entry holds a passive reference, take our own. */
 		refcount_inc(&sb->s_passive);

-- 
2.55.0