linux-kernel - Re: ecryptfs lock loop

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Date:	Wed, 15 Apr 2009 10:40:03 -0500
From:	Tyler Hicks <tyhicks@...ux.vnet.ibm.com>
To:	Jiri Slaby <jirislaby@...il.com>
CC:	kirkland@...onical.com, ecryptfs-devel@...ts.launchpad.net,
	linux-fsdevel@...r.kernel.org,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	xatrix101@...il.com
Subject: Re: ecryptfs lock loop

Jiri Slaby wrote:
> Hi,
> 
> one student (in CC) found out a suspicioous locking dependence in
> ecryptfs code while debugging/running a static lockdep analyzer.
> 
> I'm unable to say whether it is only a theoretical issue or a real bug,
> any ideas?

Hi Jiri - This looks to be a real bug.  Thanks for pointing it out!

> 
> Here it comes:
> 
> function ecryptfs_send_message:
> ------------------------------
> ecryptfs_daemon_hash_mux <- ecryptfs_msg_ctx_lists_mux
> (in function ecryptfs_send_message_locked)

It would be nice if we could keep from having to hold the
ecryptfs_daemon_hash_mux for the entire execution of
ecryptfs_send_message_locked().  Maybe we could just hold it in
ecryptfs_send_message(), while it calls ecryptfs_find_daemon_by_euid().
 I think we would need to grab the daemon->mux before releasing the
ecryptfs_daemon_hash_mux to make sure that a QUIT message wasn't
received and the daemon freed before we add the msg_ctx to the
daemon->msg_ctx_out_queue in ecryptfs_send_miscdev().

It would remove the first lock dependency you pointed out, but I'll need
some more time to determine if it creates another bad lock loop.  Thanks
again!

Tyler

> 
> function ecryptfs_wait_for_response:
> -----------------------------------
> cryptfs_msg_ctx_lists_mux <- msg_ctx->mux
> 
> function ecryptfs_process_response:
> ----------------------------------
> msg_ctx->mux <- ecryptfs_daemon_hash_mux
> 
> 
> <- means lock dependency

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