Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 23 Jun 2022 04:48:44 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     sunliming <kelulanainsley@gmail.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        sunliming@kylino.cn
Subject: Re: [PATCH] walk_component(): get inode in lookup_slow branch
 statement block
Message-ID: <YrPinPcHHNfv3E3B@ZenIV>
References: <20220622085146.444516-1-sunliming@kylinos.cn>
 <YrLwU27DNm0YWOvB@ZenIV>
 <CAJncD7RuTTLoRS_pzvn729_SX5Xsv6Pub44eCD_RbbANjn9joA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAJncD7RuTTLoRS_pzvn729_SX5Xsv6Pub44eCD_RbbANjn9joA@mail.gmail.com>
Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk

On Thu, Jun 23, 2022 at 11:44:29AM +0800, sunliming wrote:
> Al Viro <viro@zeniv.linux.org.uk> 于2022年6月22日周三 18:35写道：
> >
> > On Wed, Jun 22, 2022 at 04:51:46PM +0800, sunliming wrote:
> > > The inode variable is used as a parameter by the step_into function,
> > > but is not assigned a value in the sub-lookup_slow branch path. So
> > > get the inode in the sub-lookup_slow branch path.
> >
> > Take a good look at handle_mounts() and the things it does when
> > *not* in RCU mode (i.e. LOOKUP_RCU is not set).  Specifically,
> >                 *inode = d_backing_inode(path->dentry);
> >                 *seqp = 0; /* out of RCU mode, so the value doesn't matter */
> > this part.
> >
> > IOW, the values passed to step_into() in inode/seq are overridden unless
> > we stay in RCU mode.  And if we'd been through lookup_slow(), we'd been
> > out of RCU mode since before we called step_into().
> 
> It might be more appropriate and easier to understand to do this
> before parameter passing in the top-level  walk_component function？

It's possible to fall out of RCU mode *inside* step_into(), so we need
it done there anyway.  Unfortunately ;-/