[PATCH 3/4] afs: Fix afs_get_link() to take validate_lock around afs_read_single()

[David Howells <dhowells@redhat.com>]

Fix afs_get_link() to take the validate_lock around afs_read_single() to
prevent races between multiple ->get_link() calls.

Fixes: eae9e78951bb ("afs: Use netfslib for symlinks, allowing them to be cached")
Closes: https://sashiko.dev/#/patchset/20260326104544.509518-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
---
 fs/afs/inode.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 06e25e1b12df..5207c4a003f6 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -78,10 +78,19 @@ const char *afs_get_link(struct dentry *dentry, struct inode *inode,
 		goto good;
 
 fetch:
-	ret = afs_read_single(vnode, NULL);
-	if (ret < 0)
-		return ERR_PTR(ret);
-	set_bit(AFS_VNODE_DIR_READ, &vnode->flags);
+	if (down_write_killable(&vnode->validate_lock) < 0)
+		return ERR_PTR(-ERESTARTSYS);
+	if (test_and_clear_bit(AFS_VNODE_ZAP_DATA, &vnode->flags) ||
+	    !test_bit(AFS_VNODE_DIR_READ, &vnode->flags)) {
+		ret = afs_read_single(vnode, NULL);
+		if (ret < 0) {
+			up_write(&vnode->validate_lock);
+			return ERR_PTR(ret);
+		}
+		set_bit(AFS_VNODE_DIR_READ, &vnode->flags);
+	}
+
+	up_write(&vnode->validate_lock);
 
 good:
 	folio = folioq_folio(vnode->directory, 0);