Trond Myklebust wrote:
On Wed, 2005-11-30 at 15:32 -0500, William H. Taber wrote:
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.
So exactly why does autofs4 want to hold the dir->i_sem in d_revalidate
in the first place? Can't we move any code that requires dir->i_sem to
be held into a ->lookup() method?
It's not that d_revalidate wants or doesn't want to hold the lock. The
caller of lookup_one_len is required to get the lock and this function
calls lookup_hash which calls cached_lookup which calls d_revalidate.
Trivially, if you have a d_revalidate that does something like
int autofs_revalidate(struct dentry *dentry, struct nameidata *nd)
{
d_drop(dentry);
return 0;
}
then the VFS will currently allocate a new dentry with the same name,
and call ->lookup() on it without dropping dir->i_sem. If you still need
to reference the old dentry, then put it on a private list somewhere.
That would also allow you to return the old dentry as the result of the
->lookup() operation if that is desirable.
Problem with that, as I understand it and Ian Kent knows better than I,
is that the autofs lookup code creates the dentry and fills it in
partially and marks it as waiting for mounting and wakes up the
automount demon. The demon completes the mount and finishes filling in
the dentry. So we cannot have some other lookup coming in and removing
the dentry on us. At least that is what I understand from Ian's answer
when I proposed the same sort of thing to him. Even if they end up
doing something like that in a future version of the automounter, I
would still like a simple patch that can be applied to existing systems
as an interim fix.
Will
Will
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