linux-kernel - Re: [RFC PATCH 1/3] lib/list_batch: A simple list insertion/deletion batching facility

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Date:	Wed, 27 Jan 2016 21:54:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [RFC PATCH 1/3] lib/list_batch: A simple list insertion/deletion
 batching facility

On Wed, Jan 27, 2016 at 03:22:19PM -0500, Waiman Long wrote:

> >>+	/*
> >>+	 * Put itself into the list_batch queue
> >>+	 */
> >>+	node.next  = NULL;
> >>+	node.entry = entry;
> >>+	node.cmd   = cmd;
> >>+	node.state = lb_state_waiting;
> >>+
> >Here we rely on the release barrier implied by xchg() to ensure the node
> >initialization is complete before the xchg() publishes the thing.
> >
> >But do we also need the acquire part of this barrier? From what I could
> >tell, the primitive as a whole does not imply any ordering.
> 
> I think we probably won't need the acquire part, but I don't have a non-x86
> machine that can really test out the more relaxed versions of the atomic
> ops. That is why I use the strict versions. We can always relax it later on
> with additional patches.

Yeah, I have no hardware either. But at least we should comment the bits
we do know to rely upon.


> >>+	if (!next) {
> >>+		/*
> >>+		 * The queue tail should equal to nptr, so clear it to
> >>+		 * mark the queue as empty.
> >>+		 */
> >>+		if (cmpxchg(&batch->tail, nptr, NULL) != nptr) {
> >>+			/*
> >>+			 * Queue not empty, wait until the next pointer is
> >>+			 * initialized.
> >>+			 */
> >>+			while (!(next = READ_ONCE(nptr->next)))
> >>+				cpu_relax();
> >>+		}
> >>+		/* The above cmpxchg acts as a memory barrier */
> >for what? :-)
> >
> >Also, if that cmpxchg() fails, it very much does _not_ act as one.
> >
> >I suspect you want smp_store_release() setting the state_done, just as
> >above, and then use cmpxchg_relaxed().
> 
> You are right. I did forgot about there was no memory barrier guarantee when
> cmpxchg() fails. 

> However, in that case, the READ_ONCE() and WRITE_ONCE()
> macros should still provide the necessary ordering, IMO.

READ/WRITE_ONCE() provide _no_ order what so ever. And the issue here is
that we must not do any other stores to nptr after the state_done.

> I can certainly
> change it to use cmpxchg_relaxed() and smp_store_release() instead.

That seems a safe combination and would still generate the exact same
code on x86.