linux-kernel - Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake

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Message-ID: <20150924132121.GA1814@fixme-laptop.cn.ibm.com>
Date:	Thu, 24 Sep 2015 21:21:22 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] Documentation: Remove misleading examples of the
 barriers in wake_*()

Hi Peter,

On Thu, Sep 17, 2015 at 03:01:26PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 10, 2015 at 07:55:57PM +0200, Oleg Nesterov wrote:
> > On 09/10, Boqun Feng wrote:
> > >
> > > On Wed, Sep 09, 2015 at 12:28:22PM -0700, Paul E. McKenney wrote:
> > > > My feeling is
> > > > that we should avoid saying too much about the internals of wait_event()
> > > > and wake_up().
> > 
> > I feel the same. I simply can't understand what we are trying to
> > document ;)
> 
> So I've been sitting on this for a while and figured I'd finish it now.
> 
> It are some notes on the scheduler locking and how it provides program
> order guarantees on SMP systems.
> 
> Included in it are some of the details on this subject, because a wakeup
> has two prior states that are of importance, the tasks own prior state
> and the wakeup state, both should be considered in the 'program order'
> flow.
> 

Great and very helpful ;-)

> So maybe we can reduce the description in memory-barriers to this
> 'split' program order guarantee, where a woken task must observe both
> its own prior state and its wakee state.
                              ^^^^^
I think you mean "waker" here, right?

And the waker is not necessarily the same task who set the @cond to
true, right? If so, I feel like it's really hard to *use* this 'split'
program order guarantee in other places than sleep/wakeup itself. Could
you give an example? Thank you.

Regards,
Boqun

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