linux-kernel - Re: [PATCH 4.4 13/16] ocfs2: should wait dio before inode lock in ocfs2

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Message-ID: <1512699985.18523.219.camel@codethink.co.uk>
Date:   Fri, 08 Dec 2017 02:26:25 +0000
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     alex chen <alex.chen@...wei.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        piaojun <piaojun@...wei.com>, Joseph Qi <jiangqi903@...il.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Changwei Ge <ge.changwei@....com>,
        Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Junxiao Bi <junxiao.bi@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4.4 13/16] ocfs2: should wait dio before inode lock in
 ocfs2_setattr()

On Fri, 2017-12-08 at 08:39 +0800, alex chen wrote:
> 
> On 2017/12/8 2:25, Ben Hutchings wrote:
> > On Wed, 2017-12-06 at 09:02 +0800, alex chen wrote:
> > > Hi Ben,
> > > 
> > > Thanks for your reply.
> > > 
> > > On 2017/12/5 23:49, Ben Hutchings wrote:
> > > > On Wed, 2017-11-22 at 11:12 +0100, Greg Kroah-Hartman wrote:
> > > > > 4.4-stable review patch.  If anyone has any objections,
> > > > > please let me know.
> > > > > 
> > > > > ------------------
> > > > > 
> > > > > From: alex chen <alex.chen@...wei.com>
> > > > > 
> > > > > commit 28f5a8a7c033cbf3e32277f4cc9c6afd74f05300 upstream.
> > > > > 
> > > > > we should wait dio requests to finish before inode lock in
> > > > > ocfs2_setattr(), otherwise the following deadlock will
> > > > > happen:
> > > > 
> > > > [...]
> > > > 
> > > > I looked at the kernel-doc for inode_dio_wait():
> > > > 
> > > > /**
> > > >  * inode_dio_wait - wait for outstanding DIO requests to finish
> > > >  * @inode: inode to wait for
> > > >  *
> > > >  * Waits for all pending direct I/O requests to finish so that we can
> > > >  * proceed with a truncate or equivalent operation.
> > > >  *
> > > >  * Must be called under a lock that serializes taking new references
> > > >  * to i_dio_count, usually by inode->i_mutex.
> > > >  */
> > > > 
> > > > Now that ocfs2_setattr() calls this outside of the inode locked region,
> > > > what prevents another task adding a new dio request immediately
> > > > afterward?
> > > > 
> > > 
> > > In the kernel 4.6, firstly, we use the inode_lock() in do_truncate() to
> > > prevent another bio to be issued from this node.
> > 
> > [...]
> > 
> > Yes but there seems to be a race condition - after the call to
> > inode_dio_wait() and before the call to inode_lock(), another dio
> > request can be added.

Sorry, I've been mixing up inode_lock() and ocfs2_inode_lock(). 
However:

> In the truncating file situation, the lock order is as follow:
> do_truncate()
>  inode_lock()
>  notify_change()
>   ocfs2_setattr()
>    inode_dio_wait()
>     --here it is under the protect of inode_lock(), so another dio requests
>       from another process will not be added.

only DIO reads seem to take the inode lock.

Ben.

>    ocfs2_rw_lock()
>    ocfs2_inode_lock_tracker()
>     this function is used to prevent the inode from being modified by another
>     nodes in the cluster
>  inode_unlock()
> 
> > 
> > Ben.
> > 
> 
> 
-- 
Ben Hutchings
Software Developer, Codethink Ltd.