linux-kernel - Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive

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Date:	Fri, 29 Mar 2013 10:57:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Rik van Riel <riel@...riel.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	torvalds@...ux-foundation.org, davidlohr.bueso@...com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	hhuang@...hat.com, jason.low2@...com, lwoodman@...hat.com,
	chegu_vinod@...com, Dave Jones <davej@...hat.com>,
	benisty.e@...il.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive

On Thu, 2013-03-28 at 19:50 -0700, Michel Lespinasse wrote:
> So, there are a few things I don't like about spin_unlock_wait():
> 
> 1- From a lock ordering point of view, it is strictly equivalent to
> taking the lock and then releasing it - and yet, lockdep won't catch
> any deadlocks that involve spin_unlock_wait. (Not your fault here,
> this should be fixed as a separate change in lockdep. I manually
> looked at the lock ordering here and found it safe).

Ooh, I never noticed that, but indeed this shouldn't be hard to cure.

> 2- With the current ticket lock implementation, a stream of lockers
> can starve spin_unlock_wait() forever. Once again, not your fault and
> I suspect this could be fixed - I expect spin_unlock_wait() callers
> actually only want to know that the lock has been passed on, not that
> it actually got to an unlocked state.

I suppose the question is do we want to fix it or have both semantics
and use lock+unlock where appropriate.


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