[PATCH 4/4] xfs_scrub: handle concurrent directory updates during name scan

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

The name scanner in xfs_scrub cannot lock a namespace (dirent or xattr)
and the kernel does not provide a stable cursor interface, which means
that we can see the same byte sequence multiple times during a scan.
This isn't a confusing name error since the kernel enforces uniqueness
on the byte sequence, so all we need to do here is update the old entry.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 scrub/unicrash.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)


diff --git a/scrub/unicrash.c b/scrub/unicrash.c
index de3217c2..cb0880c1 100644
--- a/scrub/unicrash.c
+++ b/scrub/unicrash.c
@@ -68,7 +68,7 @@ struct name_entry {
 
 	xfs_ino_t		ino;
 
-	/* Raw UTF8 name */
+	/* Raw dirent name */
 	size_t			namelen;
 	char			name[0];
 };
@@ -627,6 +627,20 @@ unicrash_add(
 	uc->buckets[bucket] = new_entry;
 
 	while (entry != NULL) {
+		/*
+		 * If we see the same byte sequence then someone's modifying
+		 * the namespace while we're scanning it.  Update the existing
+		 * entry's inode mapping and erase the new entry from existence.
+		 */
+		if (new_entry->namelen == entry->namelen &&
+		    !memcmp(new_entry->name, entry->name, entry->namelen)) {
+			entry->ino = new_entry->ino;
+			uc->buckets[bucket] = new_entry->next;
+			name_entry_free(new_entry);
+			*badflags = 0;
+			return;
+		}
+
 		/* Same normalization? */
 		if (new_entry->normstrlen == entry->normstrlen &&
 		    !u_strcmp(new_entry->normstr, entry->normstr) &&