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

From: Boqun Feng
Date: Thu Sep 24 2015 - 09:22:01 EST

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.


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