[RFC PATCH v2 2/5] libfs: Check dentry before locking in simple_offset_empty()

[cel@kernel.org]

From: Chuck Lever <chuck.lever@oracle.com>

Defensive change: Don't try to lock a dentry unless it is positive.
Trying to lock a negative entry will generate a refcount underflow.

The underflow has been seen only while testing.

Fixes: ecba88a3b32d ("libfs: Add simple_offset_empty()")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index bf67954b525b..c88ed15437c7 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -347,13 +347,14 @@ int simple_offset_empty(struct dentry *dentry)
 	index = DIR_OFFSET_MIN;
 	octx = inode->i_op->get_offset_ctx(inode);
 	mt_for_each(&octx->mt, child, index, LONG_MAX) {
-		spin_lock(&child->d_lock);
 		if (simple_positive(child)) {
+			spin_lock(&child->d_lock);
+			if (simple_positive(child))
+				ret = 0;
 			spin_unlock(&child->d_lock);
-			ret = 0;
-			break;
+			if (!ret)
+				break;
 		}
-		spin_unlock(&child->d_lock);
 	}
 
 	return ret;
-- 
2.47.0