linux-kernel - Re: Need lockdep help

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.44L0.0712031000480.3630-100000@iolanthe.rowland.org>
Date:	Mon, 3 Dec 2007 10:08:43 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jarek Poplawski <jarkao2@...pl>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Need lockdep help

On Mon, 3 Dec 2007, Jarek Poplawski wrote:

> > System sleep start:
> > 		down_read(notifier-chain rwsem);
> > 		call the notifier routine
> > 			down_write(&system_sleep_in_progress_rwsem);
> > 		up_read(notifier-chain rwsem);
> > 
> > System sleep end:
> > 		down_read(notifier-chain rwsem);
> > 		call the notifier routine
> > 			up_write(&system_sleep_in_progress_rwsem);
> > 		up_read(notifier-chain rwsem);
> > 
> > This creates a lockdep violation; each rwsem in turn is locked while 
> > the other is being held.  However the only way this could lead to 
> > deadlock would be if there was already a bug in the system Power 
> > Management code (overlapping notifications).
> 
> Actually, IMHO, there is no reason for any lockdep violation:
> 
> thread #1: has down_read(A); waits for #2 to down_write(B)
> thread #2: has down_write(B); never waits for #1 to down_read(A)
> 
> So, deadlock isn't possible here. If lockdep reports something else it
> should be fixed (and you'd be right to omit lockdep until this is
> done).

I think the reasoning goes the way Arjan described.  Suppose in between
#1 and #2 there is thread #3 trying to do down_write(A) and waiting for
#1.  Then thread #2 doesn't have to wait for #1 directly, but it would
have to wait for #3.

In my case the simplest answer appears to be the replace the rwsem
with something slightly more complicated (a mutex plus a boolean flag 
-- the loss of concurrency won't matter much since it isn't on a hot 
path).

Thanks for the comment.

Alan Stern

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/