cifs: fix oops while traversing open file list (try #4)

While traversing the linked list of open file handles, if the identfied
file handle is invalid, a reopen is attempted and if it fails, we
resume traversing where we stopped and cifs can oops while accessing
invalid next element, for list might have changed.

So mark the invalid file handle and attempt reopen if no
valid file handle is found in rest of the list.
If reopen fails, move the invalid file handle to the end of the list
and start traversing the list again from the begining.
Repeat this four times before giving up and returning an error if
file reopen keeps failing.

Cc: <stable@vger.kernel.org>
Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsglob.h

@@ -43,6 +43,7 @@
 
 #define CIFS_MIN_RCV_POOL 4
 
+#define MAX_REOPEN_ATT	5 /* these many maximum attempts to reopen a file */
 /*
  * default attribute cache timeout (jiffies)
  */

fs/cifs/file.c

@@ -1565,10 +1565,11 @@ struct cifsFileInfo *find_readable_file(struct cifsInodeInfo *cifs_inode,
 struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *cifs_inode,
 					bool fsuid_only)
 {
-	struct cifsFileInfo *open_file;
+	struct cifsFileInfo *open_file, *inv_file = NULL;
 	struct cifs_sb_info *cifs_sb;
 	bool any_available = false;
 	int rc;
+	unsigned int refind = 0;
 
 	/* Having a null inode here (because mapping->host was set to zero by
 	the VFS or MM) should not happen but we had reports of on oops (due to
@@ -1588,40 +1589,25 @@ struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *cifs_inode,
 
 	spin_lock(&cifs_file_list_lock);
 refind_writable:
+	if (refind > MAX_REOPEN_ATT) {
+		spin_unlock(&cifs_file_list_lock);
+		return NULL;
+	}
 	list_for_each_entry(open_file, &cifs_inode->openFileList, flist) {
 		if (!any_available && open_file->pid != current->tgid)
 			continue;
 		if (fsuid_only && open_file->uid != current_fsuid())
 			continue;
 		if (OPEN_FMODE(open_file->f_flags) & FMODE_WRITE) {
-			cifsFileInfo_get(open_file);
-
 			if (!open_file->invalidHandle) {
 				/* found a good writable file */
+				cifsFileInfo_get(open_file);
 				spin_unlock(&cifs_file_list_lock);
 				return open_file;
+			} else {
+				if (!inv_file)
+					inv_file = open_file;
 			}
-
-			spin_unlock(&cifs_file_list_lock);
-
-			/* Had to unlock since following call can block */
-			rc = cifs_reopen_file(open_file, false);
-			if (!rc)
-				return open_file;
-
-			/* if it fails, try another handle if possible */
-			cFYI(1, "wp failed on reopen file");
-			cifsFileInfo_put(open_file);
-
-			spin_lock(&cifs_file_list_lock);
-
-			/* else we simply continue to the next entry. Thus
-			   we do not loop on reopen errors.  If we
-			   can not reopen the file, for example if we
-			   reconnected to a server with another client
-			   racing to delete or lock the file we would not
-			   make progress if we restarted before the beginning
-			   of the loop here. */
 		}
 	}
 	/* couldn't find useable FH with same pid, try any available */
@@ -1629,7 +1615,30 @@ struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *cifs_inode,
 		any_available = true;
 		goto refind_writable;
 	}
+
+	if (inv_file) {
+		any_available = false;
+		cifsFileInfo_get(inv_file);
+	}
+
 	spin_unlock(&cifs_file_list_lock);
+
+	if (inv_file) {
+		rc = cifs_reopen_file(inv_file, false);
+		if (!rc)
+			return inv_file;
+		else {
+			spin_lock(&cifs_file_list_lock);
+			list_move_tail(&inv_file->flist,
+					&cifs_inode->openFileList);
+			spin_unlock(&cifs_file_list_lock);
+			cifsFileInfo_put(inv_file);
+			spin_lock(&cifs_file_list_lock);
+			++refind;
+			goto refind_writable;
+		}
+	}
+
 	return NULL;
 }