Trond Myklebust wrote:
On Tue, 2005-11-29 at 20:16 -0500, Jeff Moyer wrote:
==> Regarding [autofs] [RFC PATCH]autofs4: hang and proposed fix; [EMAIL
PROTECTED] (Ram Pai) adds:
linuxram> Autofs4 assumes that its ->revalidate() function gets called with
linuxram> the parent_dentry's_inode_semaphore released. This is true mostly
linuxram> but not in one particular case.
linuxram> Process P1 calls autofs4's ->lookup(). The lookup finds that the
linuxram> dentry does not exist. It creates a dentry and adds to the
linuxram> cache. Releases the parent's inode's semaphore and than calls
linuxram> ->revalidate().
linuxram> Process P2 meanwhile comes in and cached_lookup() gets called. It
linuxram> finds the dentry in the cache and finds ->revalidate() function
linuxram> exists. So it calls ->revalidate() holding the parent's inode's
linuxram> semaphore.
Can't we simply fix this case? It seems like it should be perfectly safe
to drop the parent's i_sem before calling revalidate in cached_lookup. In
fact, there are comments in the NFS code that would lead one to believe
that revalidate is not supposed to be called with the parent's i_sem held:
static int nfs_lookup_revalidate(struct dentry * dentry, struct nameidata *nd)
{
...
/*
* Note: we're not holding inode->i_sem and so may be racing with
* operations that change the directory. We therefore save the
* change attribute *before* we do the RPC call.
*/
Can you try out a patch which does this?
-Jeff
--- linux-2.6.14/fs/namei.c.orig 2005-11-29 20:14:30.000000000 -0500
+++ linux-2.6.14/fs/namei.c 2005-11-29 20:14:48.000000000 -0500
@@ -332,10 +332,12 @@ static struct dentry * cached_lookup(str
dentry = d_lookup(parent, name);
if (dentry && dentry->d_op && dentry->d_op->d_revalidate) {
+ up(&parent->d_inode->i_sem);
if (!dentry->d_op->d_revalidate(dentry, nd) &&
!d_invalidate(dentry)) {
dput(dentry);
dentry = NULL;
}
+ down(&parent->d_inode->i_sem);
}
return dentry;
}
Woah! Definitely not safe. NFS might not care, but the VFS will
certainly barf over that!
By dropping the dir->i_sem in cached_lookup() you are allowing 2
processes to allocate and lookup multiple dentries for the same file
inside __lookup_hash().
Cheers,
Trond
Not only is there this case, but the original premise is wrong as well.
There is a second case in which a d_revalidate function is called with
the parent i_sem and that is when it is called from inside of
lookup_one_len. What makes this tricky is that lookup_one_len is called
from nfs_sillyrename from inside of nfs_rename which is called,
naturally enough by sys_rename. The rename code is very careful about
the order in which it obtains the parent semaphores because it needs to
get two of them. It must always obtain the locks in the same order so
that does not get into a deadly embrace. If we start arbitrarily
releasing a parent semaphore in cached_lookup and taking it again after
the revalidate, we risk breaking the lock ordering and creating a deadly
embrace.
When I started writing this I thought that it would be safe for the
autofs revalidate code to release the parent semaphore because they do
not have a rename callback. But I looked again at the rename code and
it calls lookup_hash on the final source and destination files after
locking the parents so the potential for a deadly embrace still exists
unless there is some other assurance that these final lookups will never
pend waiting on the automounter in either their revalidate or lookup
routines. (Actually the requirement is that they never give up the
parent i_sem lock, but the lookup code has to give up the lock so that
the autofs demon can run and perform the mount so it amounts to the same
thing.)
The same issue exists for devfs which also releases the parent i_sem
lock so that it can wait inside its revalidation routine.
Will
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