Re: 2.6.33.5 rt23: machine lockup (nfs/autofs related?)

john stultz
2010-07-12T23:54:04Z

On Mon, 2010-07-12 at 16:37 -0700, Fernando Lopez-Lezcano wrote:
> On Fri, 2010-07-09 at 15:57 -0700, john stultz wrote:
> > So looking over it, I'm not easily seeing what else could be off.
> >
> > So Lets see if we can cut some of the guess work out of this...
> >
> > > [<c04e08e9>] ? d_materialise_unique+0xbf/0x29e
> >
> > I'm curious exactly where that is in d_materialise_unique. To find out,
> > can you find the vmlinux image in the base of the directory you built
> > the kernel you triggered this in?
> >
> > Then run:
> > # gdb ./vmlinux
> >
> > Once gdb loads:
> > (gdb) list *0xc04e08e9
> >
> > That should point to exactly where in the function we are trying to
> > acquire a previously locked lock.
>
> Finally... I did a local build in my desktop machine so I now have
> access to the full patched/compiled source tree. I confirmed that the
> patch you sent is there (moving a spin_lock one line down).
>
> This is from a different kernel (non-PAE) so the exact address is
> different from the previous report:
>
> (gdb) list *0xc04d82dd
> 0xc04d82dd is in d_materialise_unique (fs/dcache.c:2100).
> 2095 spin_lock(&aparent->d_lock);
> 2096 spin_lock(&dparent->d_lock);
> 2097 spin_lock(&dentry->d_lock);
> 2098 spin_lock(&anon->d_lock);
> 2099
> 2100 dentry->d_parent = (aparent == anon) ? dentry : aparent;
> 2101 list_del(&dentry->d_u.d_child);
> 2102 if (!IS_ROOT(dentry))
> 2103 list_add(&dentry->d_u.d_child, &dentry->d_parent->d_subdirs);
> 2104 else
>
> See below for the full dump of the BUG through the serial console in
> this particular occurrence.

Huh. I'm still baffled. Since we're blowing out on line 2098, the anon
pointer points to the alias pointer we passed in to
__d_materialise_dentry(). So that means the anon dentry is already
locked, and we've moved the obviously wrong lock operation down so it
shouldn't be held.

Hrm. Ok.. I think the line 2100 above gives us a hint: (aparent == anon)
So if that were the case, we would have already locked aparent and that
would explain the blowup.

How does it do with the following change?

thanks
-john

diff --git a/fs/dcache.c b/fs/dcache.c
index c9d21ae..8d68504 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2099,7 +2099,8 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
aparent = anon->d_parent;

/* XXX: hack */
- spin_lock(&aparent->d_lock);
+ if (aparent != anon)
+ spin_lock(&aparent->d_lock);
spin_lock(&dparent->d_lock);
spin_lock(&dentry->d_lock);
spin_lock(&anon->d_lock);
@@ -2121,7 +2122,8 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
spin_unlock(&anon->d_lock);
spin_unlock(&dentry->d_lock);
spin_unlock(&dparent->d_lock);
- spin_unlock(&aparent->d_lock);
+ if (aparent != anon)
+ spin_unlock(&aparent->d_lock);

anon->d_flags &= ~DCACHE_DISCONNECTED;
}
@@ -2159,8 +2161,8 @@ struct dentry *d_materialise_unique(struct dentry *dentry, struct inode *inode)
/* Is this an anonymous mountpoint that we could splice
* into our tree? */
if (IS_ROOT(alias)) {
- spin_lock(&alias->d_lock);
__d_materialise_dentry(dentry, alias);
+ spin_lock(&alias->d_lock);
__d_drop(alias);
goto found;
}