linux-kernel - Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups

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Date:	Sat, 31 Oct 2015 11:31:05 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:	linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Manfred Spraul <manfred@...orfullife.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	George Spelvin <linux@...izon.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] ipc/msg: Implement lockless pipelined wakeups

On Fri, 30 Oct 2015, Sebastian Andrzej Siewior wrote:
> This was just compile tested. It would be nice if somone with real
> workload could test it. Otherwise I could hack something myself and check
> if it still works.

ltp and glibc should have tests at least at the functional level
 
> -static void expunge_all(struct msg_queue *msq, int res)
> +static void expunge_all(struct msg_queue *msq, int res,
> +			struct wake_q_head *wake_q)
>  {
>  	struct msg_receiver *msr, *t;
>  
>  	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
> -		msr->r_msg = NULL; /* initialize expunge ordering */

Don't we need to keep that NULL init? I might be missing something.

> -		wake_up_process(msr->r_tsk);
> -		/*
> -		 * Ensure that the wakeup is visible before setting r_msg as
> -		 * the receiving end depends on it: either spinning on a nil,
> -		 * or dealing with -EAGAIN cases. See lockless receive part 1
> -		 * and 2 in do_msgrcv().
> -		 */
> -		smp_wmb(); /* barrier (B) */

Please keep the comment intact and fix it as it documents the mechanism
including the barriers. The barrier is now inside of wake_q_add().

> +
> +		wake_q_add(wake_q, msr->r_tsk);
>  		msr->r_msg = ERR_PTR(res);


> -static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
> +static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg,
> +				 struct wake_q_head *wake_q)
>  {
>  	struct msg_receiver *msr, *t;
>  
> @@ -577,27 +576,13 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
>  
>  			list_del(&msr->r_list);
>  			if (msr->r_maxsize < msg->m_ts) {
> -				/* initialize pipelined send ordering */
> -				msr->r_msg = NULL;

See above.

> -				wake_up_process(msr->r_tsk);
> -				/* barrier (B) see barrier comment below */
> -				smp_wmb();

Please do not remove barrier documentation. Update it.

> +				wake_q_add(wake_q, msr->r_tsk);
>  				msr->r_msg = ERR_PTR(-E2BIG);
>  			} else {
> -				msr->r_msg = NULL;
>  				msq->q_lrpid = task_pid_vnr(msr->r_tsk);
>  				msq->q_rtime = get_seconds();
> -				wake_up_process(msr->r_tsk);
> -				/*
> -				 * Ensure that the wakeup is visible before
> -				 * setting r_msg, as the receiving can otherwise
> -				 * exit - once r_msg is set, the receiver can
> -				 * continue. See lockless receive part 1 and 2
> -				 * in do_msgrcv(). Barrier (B).
> -				 */

Ditto.

> -				smp_wmb();
> +				wake_q_add(wake_q, msr->r_tsk);
>  				msr->r_msg = msg;
> -
>  				return 1;
>  			}
>  		}

> @@ -932,57 +919,25 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
>  		rcu_read_lock();
>  
>  		/* Lockless receive, part 2:
> -		 * Wait until pipelined_send or expunge_all are outside of
> -		 * wake_up_process(). There is a race with exit(), see
> -		 * ipc/mqueue.c for the details. The correct serialization
> -		 * ensures that a receiver cannot continue without the wakeup
> -		 * being visibible _before_ setting r_msg:
> +		 * The work in pipelined_send() and expunge_all():
> +		 * - Set pointer to message
> +		 * - Queue the receiver task for later wakeup
> +		 * - Wake up the process after the lock is dropped.
>  		 *
> -		 * CPU 0                             CPU 1
> -		 * <loop receiver>
> -		 *   smp_rmb(); (A) <-- pair -.      <waker thread>
> -		 *   <load ->r_msg>           |        msr->r_msg = NULL;
> -		 *                            |        wake_up_process();
> -		 * <continue>                 `------> smp_wmb(); (B)
> -		 *                                     msr->r_msg = msg;
> -		 *
> -		 * Where (A) orders the message value read and where (B) orders
> -		 * the write to the r_msg -- done in both pipelined_send and
> -		 * expunge_all.
> +		 * Should the process wake up before this wakeup (due to a
> +		 * signal) it will either see the message and continue …

Please update the barrier documentation so it matches the new code.

>  		 */
> -		for (;;) {
> -			/*
> -			 * Pairs with writer barrier in pipelined_send
> -			 * or expunge_all.
> -			 */
> -			smp_rmb(); /* barrier (A) */
> -			msg = (struct msg_msg *)msr_d.r_msg;
> -			if (msg)
> -				break;
>  
> -			/*
> -			 * The cpu_relax() call is a compiler barrier
> -			 * which forces everything in this loop to be
> -			 * re-loaded.
> -			 */
> -			cpu_relax();
> -		}

Are you sure that this is not longer required? If so then please
explain and update the documentation accordingly.

Thanks,

	tglx