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

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Date:	Thu, 28 Jan 2016 19:35:26 +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 Thu, Jan 28, 2016 at 11:45:40AM -0500, Waiman Long wrote:
> Using xchg_release() looks OK to me. As this feature is enabled on x86 only
> for this patch, we can make the change and whoever enabling it for other
> architectures that have a real release function will have to test it.

Ah, I was more thinking about:

	/*
	 * We rely on the memory barrier implied by xchg() below to
	 * ensure the node initialization is complete before its
	 * published.
	 */

And then use xchg() like you already do.


> >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 thought if those macros are accessing the same cacheline, the compiler
> won't change the ordering and the hardware will take care of the proper
> ordering. Anyway, I do intended to change to use smp_store_release() for
> safety.

The macros use a volatile cast, and that ensures the compiler must emit
the store and must emit it as a single store. I'm not 100% sure on the
rules of the compiler reordering volatile accesses, they are not a
compiler barrier.