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Date: Fri, 29 Mar 2013 06:59:41 -0700
Message-ID: <CANN689HTXusmGcLNByvmSQt0RGaCieZ3=YBJ-BSoeXXBO36=Xw@mail.gmail.com>
Subject: Re: [PATCH v3 -mm -next] ipc,sem: fix lockdep false positive
From: Michel Lespinasse <walken@google.com>
To: Rik van Riel <riel@surriel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        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>
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On Fri, Mar 29, 2013 at 6:55 AM, Rik van Riel <riel@surriel.com> wrote:
> On Thu, 28 Mar 2013 19:50:47 -0700
> Michel Lespinasse <walken@google.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@infradead.org>
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Rik van Riel <riel@redhat.com>

Looks good.

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

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