On Sun, Oct 11, 2015 at 05:40:44PM -0700, Paul E. McKenney wrote:
> On Sun, Oct 11, 2015 at 11:26:40PM +0800, Boqun Feng wrote:
> > On Tue, Oct 06, 2015 at 06:06:50PM +0200, Peter Zijlstra wrote:
> > > 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.
> >
> > Got it. So at this point, I think it's better to remove the entire
> > "Sleep and wake-up functions" section in memory-barriers.txt. Because
> > this order guarantee is not for other users except sleep/wakeup. Any
> > concern, Paul?
>
> The concern I have with just removing it is that it is all too easy for
> people to assume that they provide ordering. So we should at least have
Understood.
But, IMO, the position of this section is already misleading:
(*) Implicit kernel memory barriers.
- Locking functions.
- Interrupt disabling functions.
->- Sleep and wake-up functions.<-
- Miscellaneous functions.
I read it as that sleep and wake-up functions provide some kernel memory
barriers which we can use *externally*(outside sleep/wakeup themselves).
So how about something like:
(*) Barriers only for internal use
- Sleep and wake-up functions.
Regards,
Boqun
> a section stating clearly that ordering is not guaranteed without help
> from locks, release-acquire, explicit memory barriers, etc.
>
> Thanx, Paul
>
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