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

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Date:	Tue, 26 Mar 2013 14:19:25 +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 -mm -next] ipc,sem: fix lockdep false positive

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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