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

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Message-ID: <CANN689G9Xt6Hmpg70x3LMy-KtccD5EvdjOW4SF+d6PsJCwK0tg@mail.gmail.com>
Date:	Wed, 27 Mar 2013 04:22:23 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Wed, Mar 27, 2013 at 1:42 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2013-03-26 at 11:19 -0400, Rik van Riel wrote:
>> > Maybe something like:
>> >
>> > void sma_lock(struct sem_array *sma) /* global */
>> > {
>> >       int i;
>> >
>> >       sma->global_locked = 1;
>> >       smp_wmb(); /* can we merge with the LOCK ? */
>> >       spin_lock(&sma->global_lock);
>> >
>> >       /* wait for all local locks to go away */
>> >       for (i = 0; i < sma->sem_nsems; i++)
>> >               spin_unlock_wait(&sem->sem_base[i]->lock);
>> > }
>> >
>> > void sma_lock_one(struct sem_array *sma, int nr) /* local */
>> > {
>> >       smp_rmb(); /* pairs with wmb in sma_lock() */
>> >       if (unlikely(sma->global_locked)) { /* wait for global lock */
>> >               while (sma->global_locked)
>> >                       spin_unlock_wait(&sma->global_lock);
>> >       }
>> >       spin_lock(&sma->sem_base[nr]->lock);
>> > }
>
> I since realized there's an ordering problem with ->global_locked, we
> need to use spin_is_locked() or somesuch.
>
> Two competing sma_lock() operations will screw over the separate
> variable.
>
>>
>> > This still has the problem of a non-preemptible section of
>> O(sem_nsems)
>> > (with the avg wait-time on the local lock). Could we make the global
>> > lock a sleeping lock?
>>
>> Not without breaking your scheme above :)
>
> How would making sma->global_lock a mutex wreck anything?

I don't remember the details (rik probably will), but rcu is also
already involved, so there is a non trivial chance that it would...

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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