From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Oleg Nesterov <oleg@redhat.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: Sun, 11 Oct 2015 17:40:44 -0700 [thread overview]
Message-ID: <20151012004044.GZ3910@linux.vnet.ibm.com> (raw)
In-Reply-To: <20151011152640.GC27351@fixme-laptop.cn.ibm.com>
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
a section stating clearly that ordering is not guaranteed without help
from locks, release-acquire, explicit memory barriers, etc.
Thanx, Paul
next prev parent reply other threads:[~2015-10-12 0:40 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-08 1:14 [PATCH] Documentation: Remove misleading examples of the barriers in wake_*() Boqun Feng
2015-09-09 19:28 ` Paul E. McKenney
2015-09-10 2:16 ` Boqun Feng
2015-09-10 17:55 ` Oleg Nesterov
2015-09-11 16:59 ` Boqun Feng
2015-09-17 13:01 ` Peter Zijlstra
2015-09-17 17:01 ` Oleg Nesterov
2015-09-18 6:49 ` Peter Zijlstra
2015-09-21 17:46 ` Oleg Nesterov
2015-10-06 16:04 ` Peter Zijlstra
2015-10-06 16:24 ` Peter Zijlstra
2015-10-06 16:35 ` Will Deacon
2015-10-06 19:57 ` Peter Zijlstra
2015-10-07 11:10 ` Peter Zijlstra
2015-10-07 15:40 ` Paul E. McKenney
2015-09-24 13:21 ` Boqun Feng
2015-10-06 16:06 ` Peter Zijlstra
2015-10-11 15:26 ` Boqun Feng
2015-10-12 0:40 ` Paul E. McKenney [this message]
2015-10-12 9:06 ` Boqun Feng
2015-10-12 11:54 ` Peter Zijlstra
2015-10-12 13:09 ` Boqun Feng
2015-10-12 16:26 ` Peter Zijlstra
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