linux-kernel - Re: [PATCH v2] ipc/mqueue: remove STATE

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Message-ID: <20150428123738.GY23123@twins.programming.kicks-ass.net>
Date:	Tue, 28 Apr 2015 14:37:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Manfred Spraul <manfred@...orfullife.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] ipc/mqueue: remove STATE_PENDING

On Mon, Apr 27, 2015 at 08:24:53PM -0700, Davidlohr Bueso wrote:
> +static inline void pipelined_send(struct wake_q_head *wake_q,
> +				  struct mqueue_inode_info *info,
>  				  struct msg_msg *message,
>  				  struct ext_wait_queue *receiver)
>  {
>  	receiver->msg = message;
>  	list_del(&receiver->list);
> +	wake_q_add(wake_q, receiver->task);
> +	/*
> +	 * Ensure that updating receiver->state is the last
> +	 * write operation: As once set, the receiver can continue,
> +	 * and if we don't have the reference count from the wake_q,
> +	 * yet, at that point we can later have a use-after-free
> +	 * condition and bogus wakeup.
> +	 */
> +	smp_wmb(); /* pairs with smp_rmb() in wq_sleep */

You have this barrier because we cannot rely on a failed cmpxchg()
actually being a full barrier, right?

>  	receiver->state = STATE_READY;
>  }


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