linux-ext4 - Re: [RFC] inode.i_opflags - Usage of two different locking schemes

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Date:   Fri, 5 Mar 2021 10:18:16 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Alexander Lochmann <alexander.lochmann@...dortmund.de>
Cc:     Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        Jan Kara <jack@...e.cz>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] inode.i_opflags - Usage of two different locking schemes

On Fri, Mar 05, 2021 at 02:10:09PM +0100, Alexander Lochmann wrote:
> Hi folks,
> 
> I've stumbled across an interesting locking scheme. It's related to struct
> inode, more precisely it is an mqueue inode.
> Our results show that inode:mqueue.i_opflags is read with i_rwsem being
> hold.
> In d_flags_for_inode, and do_inode_permission the i_lock is used to read and
> write i_opflags.
> Is this a real locking scheme? Is a lock needed to access i_opflags at all?
> What is the magic behind this contradiction?
> 
> I've put the report of the counterexamples on our webserver:
> https://ess.cs.tu-dortmund.de/lockdoc-bugs/cex-inode-mqueue.html.
> It contains the stacktraces leading to those accesses, and the locks that
> were actually held.

1)  I don't see where i_opflags is being read in ipc/mqueue.c at all,
either with or without i_rwsem.

2)  I'm not sure what this has to do with ext4?

        	    	      	       	  - Ted