Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake_*()

From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	Jonathan Corbet <corbet@lwn.net>,
	Michal Hocko <mhocko@kernel.org>,
	David Howells <dhowells@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake_*()
Date: Tue, 6 Oct 2015 18:06:50 +0200	[thread overview]
Message-ID: <20151006160650.GT3604@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20150924132121.GA1814@fixme-laptop.cn.ibm.com>

On Thu, Sep 24, 2015 at 09:21:22PM +0800, Boqun Feng wrote:
> > 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?

Yes.

> And the waker is not necessarily the same task who set the @cond to
> true, right? 

It should be.

> 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.

It was not meant to be used in any other scenario; the 'split' PO really
is part of the whole sleep/wakeup. It does not apply to anything else.