linux-kernel - Re: NFS/d_splice

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Message-ID: <20160603042648.GN14480@ZenIV.linux.org.uk>
Date:	Fri, 3 Jun 2016 05:26:48 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Drokin <green@...uxhacker.ru>
Cc:	"J. Bruce Fields" <bfields@...hat.com>, linux-nfs@...r.kernel.org,
	"<linux-kernel@...r.kernel.org> Mailing List" 
	<linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>
Subject: Re: NFS/d_splice_alias breakage

On Thu, Jun 02, 2016 at 11:43:59PM -0400, Oleg Drokin wrote:

> > Which of the call sites had that been and how does one reproduce that fun?
> > If you feel that posting a reproducer in the open is a bad idea, just send
> > it off-list...
> 
> This is fs/nfs/dir.c::nfs_lookup() right after no_entry label.

Bloody hell...  Was that sucker hashed on the entry into nfs_lookup()?
If we get that with call as a method, we are really fucked.

<greps>

Ho-hum...  One of the goto no_open; in nfs_atomic_open()?  That would
mean a stale negative dentry in dcache that has escaped revalidation...
Wait a minute, didn't nfs ->d_revalidate() skip everything in some
cases, leaving it to ->atomic_open()?

Looks like the right thing to do would be to do d_drop() at no_open:,
just before we call nfs_lookup().  If not earlier, actually...  How
about the following?

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index aaf7bd0..6e3a6f4 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1536,9 +1536,9 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
 		err = PTR_ERR(inode);
 		trace_nfs_atomic_open_exit(dir, ctx, open_flags, err);
 		put_nfs_open_context(ctx);
+		d_drop(dentry);
 		switch (err) {
 		case -ENOENT:
-			d_drop(dentry);
 			d_add(dentry, NULL);
 			nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
 			break;