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  1. 30 Apr, 2009 2 commits
    • Louis Rilling's avatar
      configfs: Rework configfs_depend_item() locking and make lockdep happy · 420118ca
      Louis Rilling authored 
      configfs_depend_item() recursively locks all inodes mutex from configfs root to
the target item, which makes lockdep unhappy. The purpose of this recursive
locking is to ensure that the item tree can be safely parsed and that the target
item, if found, is not about to leave.

This patch reworks configfs_depend_item() locking using configfs_dirent_lock.
Since configfs_dirent_lock protects all changes to the configfs_dirent tree, and
protects tagging of items to be removed, this lock can be used instead of the
inodes mutex lock chain.
This needs that the check for dependents be done atomically with
CONFIGFS_USET_DROPPING tagging.

Now lockdep looks happy with configfs.

[ Lifted the setting of s_type into configfs_new_dirent() to satisfy the
  atomic setting of CONFIGFS_USET_CREATING  -- Joel ]
Signed-off-by: default avatarLouis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: default avatarJoel Becker <joel.becker@oracle.com>
      420118ca
    • Louis Rilling's avatar
      configfs: Silence lockdep on mkdir() and rmdir() · e74cc06d
      Louis Rilling authored 
      When attaching default groups (subdirs) of a new group (in mkdir() or
in configfs_register()), configfs recursively takes inode's mutexes
along the path from the parent of the new group to the default
subdirs. This is needed to ensure that the VFS will not race with
operations on these sub-dirs. This is safe for the following reasons:

- the VFS allows one to lock first an inode and second one of its
  children (The lock subclasses for this pattern are respectively
  I_MUTEX_PARENT and I_MUTEX_CHILD);
- from this rule any inode path can be recursively locked in
  descending order as long as it stays under a single mountpoint and
  does not follow symlinks.

Unfortunately lockdep does not know (yet?) how to handle such
recursion.

I've tried to use Peter Zijlstra's lock_set_subclass() helper to
upgrade i_mutexes from I_MUTEX_CHILD to I_MUTEX_PARENT when we know
that we might recursively lock some of their descendant, but this
usage does not seem to fit the purpose of lock_set_subclass() because
it leads to several i_mutex locked with subclass I_MUTEX_PARENT by
the same task.

>From inside configfs it is not possible to serialize those recursive
locking with a top-level one, because mkdir() and rmdir() are already
called with inodes locked by the VFS. So using some
mutex_lock_nest_lock() is not an option.

I am proposing two solutions:
1) one that wraps recursive mutex_lock()s with
   lockdep_off()/lockdep_on().
2) (as suggested earlier by Peter Zijlstra) one that puts the
   i_mutexes recursively locked in different classes based on their
   depth from the top-level config_group created. This
   induces an arbitrary limit (MAX_LOCK_DEPTH - 2 == 46) on the
   nesting of configfs default groups whenever lockdep is activated
   but this limit looks reasonably high. Unfortunately, this also
   isolates VFS operations on configfs default groups from the others
   and thus lowers the chances to detect locking issues.

Nobody likes solution 1), which I can understand.

This patch implements solution 2). However lockdep is still not happy with
configfs_depend_item(). Next patch reworks the locking of
configfs_depend_item() and finally makes lockdep happy.

[ Note: This hides a few locking interactions with the VFS from lockdep.
  That was my big concern, because we like lockdep's protection.  However,
  the current state always dumps a spurious warning.  The locking is
  correct, so I tell people to ignore the warning and that we'll keep
  our eyes on the locking to make sure it stays correct.  With this patch,
  we eliminate the warning.  We do lose some of the lockdep protections,
  but this only means that we still have to keep our eyes on the locking.
  We're going to do that anyway.  -- Joel ]
Signed-off-by: default avatarLouis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: default avatarJoel Becker <joel.becker@oracle.com>
      e74cc06d
  3. 21 Apr, 2009 1 commit
  5. 20 Apr, 2009 11 commits
  7. 19 Apr, 2009 21 commits
  9. 18 Apr, 2009 5 commits
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