linux-kernel - Re: [PATCH v3 -mm -next] ipc,sem: fix lockdep false positive

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Message-ID: <CANN689HTXusmGcLNByvmSQt0RGaCieZ3=YBJ-BSoeXXBO36=Xw@mail.gmail.com>
Date:	Fri, 29 Mar 2013 06:59:41 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 v3 -mm -next] ipc,sem: fix lockdep false positive

On Fri, Mar 29, 2013 at 6:55 AM, Rik van Riel <riel@...riel.com> wrote:
> On Thu, 28 Mar 2013 19:50:47 -0700
> Michel Lespinasse <walken@...gle.com> wrote:
>
>> This is IMO where the spin_unlock_wait(&sma->sem_perm.lock) would
>> belong - right before the goto again.
>
> Here is the slightly more optimistic (and probably more readable)
> version of the patch:
>
> ---8<---
> Unfortunately the locking scheme originally proposed has false positives
> with lockdep.  This can be fixed by changing the code to only ever take
> one lock, and making sure that other relevant locks are not locked, before
> entering a critical section.
>
> For the "global lock" case, this is done by taking the sem_array lock,
> and then (potentially) waiting for all the semaphore's spinlocks to be
> unlocked.
>
> For the "local lock" case, we wait on the sem_array's lock to be free,
> before taking the semaphore local lock. To prevent races, we need to
> check again after we have taken the local lock.
>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Rik van Riel <riel@...hat.com>

Looks good.

Reviewed-by: Michel Lespinasse <walken@...gle.com>

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