Message-ID: <1364551075.5053.82.camel@laptop>
Subject: Re: [PATCH v2 -mm -next] ipc,sem: fix lockdep false positive
From: Peter Zijlstra <peterz@infradead.org>
To: Michel Lespinasse <walken@google.com>
Cc: Rik van Riel <riel@surriel.com>, Sasha Levin <sasha.levin@oracle.com>,
        torvalds@linux-foundation.org, davidlohr.bueso@hp.com,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        hhuang@redhat.com, jason.low2@hp.com, lwoodman@redhat.com,
        chegu_vinod@hp.com, Dave Jones <davej@redhat.com>, benisty.e@gmail.com,
        Ingo Molnar <mingo@redhat.com>
Date: Fri, 29 Mar 2013 10:57:55 +0100
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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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