linux-kernel - Re: [PATCH 05/20] afs: Implement @sys substitution handling

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Message-ID: <22233.1523002135@warthog.procyon.org.uk>
Date:   Fri, 06 Apr 2018 09:08:55 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     dhowells@...hat.com, torvalds@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/20] afs: Implement @sys substitution handling

Al Viro <viro@...IV.linux.org.uk> wrote:

> > +static struct dentry *afs_lookup_atsys(struct inode *dir, struct dentry *dentry,
> > +				       struct key *key)
> > +{
> 
> > +		ret = lookup_one_len(buf, parent, len);
> 
> Er...  Parent is locked only shared here and lookup_one_len() seriously
> depends upon exclusive lock.  As it is, race with lookup of the full name
> will mess the things up very badly.

How should it be done?  Do I have to use d_alloc_parallel(), analogous to
lookup_slow() without taking the rwsem again?

David

PS: Can you stick a banner comment on d_alloc_parallel() describing it?