Re: [PATCH 4.4 13/16] ocfs2: should wait dio before inode lock in ocfs2_setattr()

[Ben Hutchings]

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.ÂÂIf anyone has any objections,
> > > > > please let me know.
> > > > > 
> > > > > ------------------
> > > > > 
> > > > > From: alex chen <alex.chen@xxxxxxxxxx>
> > > > > 
> > > > > 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():
> > > > 
> > > > /**
> > > > Â* inode_dio_wait - wait for outstanding DIO requests to finish
> > > > Â* @inode: inode to wait for
> > > > Â*
> > > > Â* Waits for all pending direct I/O requests to finish so that we can
> > > > Â* proceed with a truncate or equivalent operation.
> > > > Â*
> > > > Â* Must be called under a lock that serializes taking new references
> > > > Â* to i_dio_count, usually by inode->i_mutex.
> > > > Â*/
> > > > 
> > > > 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()
> Âinode_lock()
> Ânotify_change()
> Â ocfs2_setattr()
> ÂÂÂinode_dio_wait()
> ÂÂÂÂ--here it is under the protect of inode_lock(), so another dio requests
> ÂÂÂÂÂÂfrom another process will not be added.

only DIO reads seem to take the inode lock.

Ben.

> ÂÂÂocfs2_rw_lock()
> ÂÂÂocfs2_inode_lock_tracker()
> ÂÂÂÂthis function is used to prevent the inode from being modified by another
> ÂÂÂÂnodes in the cluster
> Âinode_unlock()
> 
> > 
> > Ben.
> > 
> 
> 
-- 
Ben Hutchings
Software Developer, Codethink Ltd.