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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>, Christoph Hellwig <hch@lst.de>,
	Jens Axboe <axboe@kernel.dk>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	yukuai3@huawei.com, will@kernel.org
Subject: Re: [PATCH] block: ensure the memory order between bi_private and bi_status
Date: Mon, 19 Jul 2021 11:09:47 -0700	[thread overview]
Message-ID: <20210719180947.GZ4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210716101929.GD4717@worktop.programming.kicks-ass.net>

On Fri, Jul 16, 2021 at 12:19:29PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 16, 2021 at 05:02:33PM +0800, Hou Tao wrote:
> 
> > > Cachelines don't guarantee anything, you can get partial forwards.
> > 
> > Could you please point me to any reference ? I can not google
> > 
> > any memory order things by using "partial forwards".
> 
> I'm not sure I have references, but there are CPUs that can do, for
> example, store forwarding at a granularity below cachelines (ie at
> register size).
> 
> In such a case a CPU might observe the stored value before it is
> committed to memory.

There have been examples of systems with multiple hardware threads
per core, but where the hardware threads share a store buffer.
In this case, the other threads in the core might see a store before
it is committed to a cache line.

As you might imagine, the hardware implementation of memory barriers
in such a system is tricky.  But not impossible.

							Thanx, Paul

  reply	other threads:[~2021-07-19 18:12 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-01 11:35 [PATCH] block: ensure the memory order between bi_private and bi_status Hou Tao
2021-07-07  6:29 ` Hou Tao
2021-07-13  1:14   ` Hou Tao
2021-07-15  7:01 ` Christoph Hellwig
2021-07-15  8:13   ` Peter Zijlstra
2021-07-16  9:02     ` Hou Tao
2021-07-16 10:19       ` Peter Zijlstra
2021-07-19 18:09         ` Paul E. McKenney [this message]
2021-07-19 18:16   ` Paul E. McKenney

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