Message-ID: <1364303965.5053.29.camel@laptop>
Subject: Re: [PATCH -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: Tue, 26 Mar 2013 14:19:25 +0100
In-Reply-To: <CANN689FdtXs0-VKxwWHQKgnnSvcvu5hHDMmGOe0H+bdHA4BhXg@mail.gmail.com>
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On Mon, 2013-03-25 at 14:42 -0700, Michel Lespinasse wrote:
> depth nesting here...
> Adding Peter & Ingo for advice about how to proceed

> > +++ b/ipc/sem.c
> > @@ -357,7 +357,7 @@ static inline int sem_lock(struct sem_array
> *sma, struct sembuf *sops,
> >                 spin_lock(&sma->sem_perm.lock);
> >                 for (i = 0; i < sma->sem_nsems; i++) {
> >                         struct sem *sem = sma->sem_base + i;
> > -                       spin_lock(&sem->lock);
> > +                       spin_lock_nested(&sem->lock,
> SINGLE_DEPTH_NESTING);
> >                 }
> >                 locknum = -1;
> >         }

Right, so as walken said, this isn't going to work right.

I need a little more information as I've not really paid much attention
to this stuff. Firstly, is there a limit to sem_nsems or is this a
random user specified number? Secondly do we care about lock order at
all, or is array order the only order that counts?


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