[BACKPORT 4.4.y 20/25] Hang/soft lockup in d_invalidate with simultaneous calls

[Arnd Bergmann <arnd@arndb.de>]

From: Al Viro <viro@ZenIV.linux.org.uk>

It's not hard to trigger a bunch of d_invalidate() on the same
dentry in parallel.  They end up fighting each other - any
dentry picked for removal by one will be skipped by the rest
and we'll go for the next iteration through the entire
subtree, even if everything is being skipped.  Morevoer, we
immediately go back to scanning the subtree.  The only thing
we really need is to dissolve all mounts in the subtree and
as soon as we've nothing left to do, we can just unhash the
dentry and bugger off.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit 81be24d263dbeddaba35827036d6f6787a59c2c3)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 fs/dcache.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 9ffe60702299..cb554e406545 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1510,7 +1510,7 @@ static void check_and_drop(void *_data)
 {
 	struct detach_data *data = _data;
 
-	if (!data->mountpoint && !data->select.found)
+	if (!data->mountpoint && list_empty(&data->select.dispose))
 		__d_drop(data->select.start);
 }
 
@@ -1552,17 +1552,15 @@ void d_invalidate(struct dentry *dentry)
 
 		d_walk(dentry, &data, detach_and_collect, check_and_drop);
 
-		if (data.select.found)
+		if (!list_empty(&data.select.dispose))
 			shrink_dentry_list(&data.select.dispose);
+		else if (!data.mountpoint)
+			return;
 
 		if (data.mountpoint) {
 			detach_mounts(data.mountpoint);
 			dput(data.mountpoint);
 		}
-
-		if (!data.mountpoint && !data.select.found)
-			break;
-
 		cond_resched();
 	}
 }
-- 
2.20.0