Re: v4.2-rc dcache regression, probably 75a6f82a0d10

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jul 31, 2015 at 03:52:38PM -0700, Linus Torvalds wrote:

> Is that correct? Maybe, I haven't checked. And maybe it's a big bad
> bug. Regardless, it sure as hell isn't just changing the order of the
> access to those fields. That "DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU"
> clearing came from __d_instantiate(), but now it hits __d_obtain_alias
> too.

Actually, the shit had hit the fan earlier.  Look: in
commit b18825a7c8e37a7cf6abb97a12a6ad71af160de7
Author: David Howells <dhowells@redhat.com>
Date:   Thu Sep 12 19:22:53 2013 +0100

    VFS: Put a small type field into struct dentry::d_flags

we have this:
@@ -1823,7 +1794,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
                        if (err)
                                return err;
                }
-               if (!can_lookup(nd->inode)) {
+               if (!d_is_directory(nd->path.dentry)) {
                        err = -ENOTDIR; 
                        break;
                }

And that has turned the check done to an inode that *was* ours at some
point (i.e. fetching it had been followed by checking that ->d_seq had
been still valid) into something completely unprotected.  Suppose we
are in lazy mode and somebody had evicted nd->path.dentry after we'd looked
it up and before that check.  Sure, its ->d_seq had been bumped by that,
and we would've failed anyway.  With ECHILD.  Which, unlike ENOTDIR, is
"repeat in non-lazy mode".

AFAICS, that's where the problem is.  It affects only RCU mode and only
the places where dentry isn't pinned.  That place in link_path_walk() is
trivial - we just need to do
                if (unlikely(!d_can_lookup(nd->path.dentry))) {
			if (nd->flags & LOOKUP_RCU) {
				if (unlazy_walk(nd, NULL, 0))
					return -ECHILD;
			}
			return -ENOTDIR;
		}
there.  AFAICS, other places of that sort are not a problem anymore.

Folks, could you check if this fixes the problems you are seeing?

diff --git a/fs/namei.c b/fs/namei.c
index ae4e4c1..b16c3a7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1954,7 +1954,11 @@ OK:
 				continue;
 			}
 		}
-		if (unlikely(!d_can_lookup(nd->path.dentry)))
+		if (unlikely(!d_can_lookup(nd->path.dentry))) {
+			if (nd->flags & LOOKUP_RCU) {
+				if (unlazy_walk(nd, NULL, 0))
+					return -ECHILD;
+			}
 			return -ENOTDIR;
 	}
 }