linux-kernel - Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

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Message-ID: <20141121200717.GB26651@t510.redhat.com>
Date:	Fri, 21 Nov 2014 15:07:18 -0500
From:	Rafael Aquini <aquini@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr@...com>
Subject: Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma
 initialization

On Fri, Nov 21, 2014 at 02:52:26PM -0500, Rik van Riel wrote:
> When manipulating just one semaphore with semop, sem_lock only takes that
> single semaphore's lock. This creates a problem during initialization of
> the semaphore array, when the data structures used by sem_lock have not
> been set up yet. The sma->lock is already held by newary, and we just
> have to make sure everything else waits on that lock during initialization.
> 
> Luckily it is easy to make sem_lock wait on the sma->lock, by pretending
> there is a complex operation in progress while the sma is being initialized.
> 
> The newary function already zeroes sma->complex_count before unlocking
> the sma->lock.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  ipc/sem.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 454f6c6..1823160 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -507,6 +507,9 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
>  		return retval;
>  	}
>  
> +	/* Ensures sem_lock waits on &sma->lock until sma is ready. */
> +	sma->complex_count = 1;
> +
>  	id = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);
>  	if (id < 0) {
>  		ipc_rcu_putref(sma, sem_rcu_free);

Acked-by: Rafael Aquini <aquini@...hat.com>
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