linux-kernel - Re: [PATCH 10/13] VFS/namei: handle LOOKUP_RCU in page_follow_link

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Message-ID: <20150319234625.GT29656@ZenIV.linux.org.uk>
Date:	Thu, 19 Mar 2015 23:46:28 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	NeilBrown <neilb@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/13] VFS/namei: handle LOOKUP_RCU in
 page_follow_link_light.

On Fri, Mar 20, 2015 at 09:38:33AM +1100, NeilBrown wrote:

> Ahhh - that's what nd->inode is for.  I wondered.
> 
> Am I correct in thinking that dentry->d_inode can only become NULL - it cannot
> then become some other inode?

It can - consider somebody doing mkdir on that name right under you.
_All_ we are guaranteed is that at some moment nd->inode matched the
pathname this far and so was (at the same moment) path->dentry.  We
are not promised that these inode and dentry will remain associated
with each other, etc.

We ought to check ->d_seq after checking ->d_flags, BTW.  _That_ will confirm
that inode remained corresponding to that dentry until the time we'd
observed d_is_symlink(dentry), i.e. make sure that inode *is* a symlink one.

And yes, we probably would have to pass dentry and inode separately, more's
the pity.
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