linux-kernel - Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake

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Message-Id: <20170609124554.GF3721@linux.vnet.ibm.com>
Date:   Fri, 9 Jun 2017 05:45:54 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Krister Johansen <kjlx@...pleofstupid.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH tip/sched/core] Add comments to aid in safer usage of
 swake_up.

On Fri, Jun 09, 2017 at 09:19:57AM +0200, Peter Zijlstra wrote:
> On Thu, Jun 08, 2017 at 08:25:46PM -0700, Krister Johansen wrote:
> > The behavior of swake_up() differs from that of wake_up(), and from the
> > swake_up() that came from RT linux. A memory barrier, or some other
> > synchronization, is needed prior to a swake_up so that the waiter sees
> > the condition set by the waker, and so that the waker does not see an
> > empty wait list.
> 
> Urgh.. let me stare at that. But it sounds like the wrong solution since
> we wanted to keep the wait and swait APIs as close as possible.

But don't they both need some sort of ordering, be it memory barriers or
locking, to handle the case where the wait/swait doesn't actually sleep?

							Thanx, Paul