linux-kernel - [PATCH 3.14 029/130] dealing with the rest of shrink_dentry

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Message-Id: <20160301234500.791010389@linuxfoundation.org>
Date:	Tue, 01 Mar 2016 23:50:59 +0000
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	<linux-kernel@...r.kernel.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<stable@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>
Subject: [PATCH 3.14 029/130] dealing with the rest of shrink_dentry_list() livelock

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit b2b80195d8829921506880f6dccd21cabd163d0d upstream.

We have the same problem with ->d_lock order in the inner loop, where
we are dropping references to ancestors.  Same solution, basically -
instead of using dentry_kill() we use lock_parent() (introduced in the
previous commit) to get that lock in a safe way, recheck ->d_count
(in case if lock_parent() has ended up dropping and retaking ->d_lock
and somebody managed to grab a reference during that window), trylock
the inode->i_lock and use __dentry_kill() to do the rest.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/dcache.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -886,8 +886,26 @@ static void shrink_dentry_list(struct li
 		 * fragmentation.
 		 */
 		dentry = parent;
-		while (dentry && !lockref_put_or_lock(&dentry->d_lockref))
-			dentry = dentry_kill(dentry, 1);
+		while (dentry && !lockref_put_or_lock(&dentry->d_lockref)) {
+			parent = lock_parent(dentry);
+			if (dentry->d_lockref.count != 1) {
+				dentry->d_lockref.count--;
+				spin_unlock(&dentry->d_lock);
+				if (parent)
+					spin_unlock(&parent->d_lock);
+				break;
+			}
+			inode = dentry->d_inode;	/* can't be NULL */
+			if (unlikely(!spin_trylock(&inode->i_lock))) {
+				spin_unlock(&dentry->d_lock);
+				if (parent)
+					spin_unlock(&parent->d_lock);
+				cpu_relax();
+				continue;
+			}
+			__dentry_kill(dentry);
+			dentry = parent;
+		}
 	}
 }