linux-kernel - Re: [PATCH] VFS: Add back check for !inode in walk

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Date:	Thu, 7 May 2015 18:28:34 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Howells <dhowells@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] VFS: Add back check for !inode in walk_component()

On Thu, May 07, 2015 at 12:52:41PM -0400, Steven Rostedt wrote:
> 
> Commit 698934df8b45 "VFS: Combine inode checks with d_is_negative() and
> d_is_positive() in pathwalk" removed a check for inode being NULL in
> walk_component() where the type is tested. Stressing my tracefs create
> and remove instances while reading the files now triggers this:

So you get NULL ->d_inode with stale flags?  The thing is, ->d_inode
becoming NULL should happen via d_delete(), which goes throug this:
        unsigned flags = READ_ONCE(dentry->d_flags);

        flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
        WRITE_ONCE(dentry->d_flags, flags);
        smp_wmb();
        dentry->d_inode = NULL;

and after that assignment to ->d_flags you'll see d_is_negative() being
true.  OTOH, we have
                *inode = dentry->d_inode;
                if (read_seqcount_retry(&dentry->d_seq, seq))
in lookup_fast(), and read_seqcount_retry() is
{
        smp_rmb();
        return __read_seqcount_retry(s, start);
}

IOW, we have smp_rmb() between fetching ->d_inode and checking ->d_flags.

If you can reproduce that at will, could you make it dump nd->flags along with
dentry involved?
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