configfs: Fix failing mkdir() making racing rmdir() fail

When fixing the rename() vs rmdir() deadlock, we stopped locking default groups'
inodes in configfs_detach_prep(), letting racing mkdir() in default groups
proceed concurrently. This enables races like below happen, which leads to a
failing mkdir() making rmdir() fail, despite the group to remove having no
user-created directory under it in the end.

	process A: 			process B:
	/* PWD=A/B */
	mkdir("C")
	  make_item("C")
	  attach_group("C")
					rmdir("A")
					  detach_prep("A")
					    detach_prep("B")
					      error because of "C"
					  return -ENOTEMPTY
	    attach_group("C/D")
	      error (eg -ENOMEM)
	  return -ENOMEM

This patch prevents such scenarii by making rmdir() wait as long as
detach_prep() fails because a racing mkdir() is in the middle of attach_group().
To achieve this, mkdir() sets a flag CONFIGFS_USET_IN_MKDIR in parent's
configfs_dirent before calling attach_group(), and clears the flag once
attach_group() is done. detach_prep() fails with -EAGAIN whenever the flag is
hit and returns the guilty inode's mutex so that rmdir() can wait on it.

Signed-off-by: Louis Rilling <Louis.Rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h
index 5a33b58e66da..da015c12e3ea 100644
--- a/fs/configfs/configfs_internal.h
+++ b/fs/configfs/configfs_internal.h
@@ -48,6 +48,7 @@ struct configfs_dirent {
 #define CONFIGFS_USET_DIR       0x0040
 #define CONFIGFS_USET_DEFAULT   0x0080
 #define CONFIGFS_USET_DROPPING  0x0100
+#define CONFIGFS_USET_IN_MKDIR  0x0200
 #define CONFIGFS_NOT_PINNED     (CONFIGFS_ITEM_ATTR)
 
 extern spinlock_t configfs_dirent_lock;