[PATCH 5/5] fs: let a mount waiting for the old superblock to die be killed

[Josef Bacik]

When sget_fc() finds a superblock for what is being mounted and that
superblock is being shut down, grab_super() waits for SB_DEAD and sget_fc()
then looks again.  This one does have to wait: the old superblock owns the
device until ->kill_sb() is done with it.

It doesn't have to be uninterruptible though.  If the unmount is stuck, so
is every attempt to mount that device again, and none of them can be
killed.  mount(2) and fsconfig(2) can fail at this point, nothing has been
set up yet apart from a spare superblock which sget_fc() frees on its
other ways out as well.

Make the wait killable.  grab_super() now has three outcomes, so have it
return 0, -EAGAIN for "look again", or the error from the wait.

Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 fs/super.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 9f76b399f3fb..36fdf88ef88e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -673,12 +673,14 @@ EXPORT_SYMBOL(deactivate_super);
  * superblock to either become SB_BORN or for it to pass through
  * sb->kill() and be marked as SB_DEAD.
  *
- * Return: This returns true if an active reference could be acquired,
- *         false if not.
+ * Return: 0 if an active reference could be acquired, -EAGAIN if the
+ *         superblock is gone and the caller should look again,
+ *         -ERESTARTSYS if the task was killed while waiting for that.
  */
-static bool grab_super(struct super_block *sb)
+static int grab_super(struct super_block *sb)
 {
 	bool locked;
+	int ret;
 
 	refcount_inc(&sb->s_passive);
 	spin_unlock(&sb_lock);
@@ -686,13 +688,13 @@ static bool grab_super(struct super_block *sb)
 	if (locked) {
 		if (atomic_inc_not_zero(&sb->s_active)) {
 			put_super(sb);
-			return true;
+			return 0;
 		}
 		super_unlock_excl(sb);
 	}
-	wait_var_event(&sb->s_flags, super_flags(sb, SB_DEAD));
+	ret = wait_var_event_killable(&sb->s_flags, super_flags(sb, SB_DEAD));
 	put_super(sb);
-	return false;
+	return ret ? ret : -EAGAIN;
 }
 
 /*
@@ -960,9 +962,12 @@ struct super_block *sget_fc(struct fs_context *fc,
 			warnfc(fc, "reusing existing filesystem in another namespace not allowed");
 		return ERR_PTR(-EBUSY);
 	}
-	if (!grab_super(old))
+	err = grab_super(old);
+	if (err == -EAGAIN)
 		goto retry;
 	destroy_unused_super(s);
+	if (err)
+		return ERR_PTR(err);
 	return old;
 }
 EXPORT_SYMBOL(sget_fc);

-- 
2.55.0