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

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Message-ID: <20170609071957.GJ8337@worktop.programming.kicks-ass.net>
Date:   Fri, 9 Jun 2017 09:19:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Krister Johansen <kjlx@...pleofstupid.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        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 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.