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

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Date:	Mon, 25 Mar 2013 14:42:17 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	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, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -mm -next] ipc,sem: fix lockdep false positive

On Mon, Mar 25, 2013 at 1:38 PM, Rik van Riel <riel@...riel.com> wrote:
> On Mon, 25 Mar 2013 16:21:22 -0400
> Sasha Levin <sasha.levin@...cle.com> wrote:
>
>> On 03/20/2013 03:55 PM, Rik van Riel wrote:
>> > Include lkml in the CC: this time... *sigh*
>> > ---8<---
>> >
>> > This series makes the sysv semaphore code more scalable,
>> > by reducing the time the semaphore lock is held, and making
>> > the locking more scalable for semaphore arrays with multiple
>> > semaphores.
>>
>> Hi Rik,
>>
>> I'm getting the following false positives from lockdep:
>
> Does this patch fix it?

I'll be surprised if it does, because we don't actually have single
depth nesting here...
Adding Peter & Ingo for advice about how to proceed
(the one solution I know would involve using arch_spin_lock() directly
to bypass the lockdep checks, but there's got to be a better way...)

> Andrew, this looks like another one for the queue...
> ---8<---
> Subject: [PATCH -mm -next] ipc,sem: fix lockdep false positive
>
> When locking all the semaphores inside a sem_array, the kernel ends up
> locking a large number of locks with identical lockdep status. This
> trips up lockdep.  Annotate the code to prevent such warnings.
>
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  ipc/sem.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 450248e..f46441a 100644
> --- a/ipc/sem.c
> +++ 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;
>         }
> @@ -558,7 +558,7 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
>         for (i = 0; i < nsems; i++) {
>                 INIT_LIST_HEAD(&sma->sem_base[i].sem_pending);
>                 spin_lock_init(&sma->sem_base[i].lock);
> -               spin_lock(&sma->sem_base[i].lock);
> +               spin_lock_nested(&sma->sem_base[i].lock, SINGLE_DEPTH_NESTING);
>         }
>
>         sma->complex_count = 0;
>

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