linux-kernel - Re: tasks-trace RCU: question about grace period forward progress

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Message-ID: <20210225230531.GW2743@paulmck-ThinkPad-P72>
Date:   Thu, 25 Feb 2021 15:05:31 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        rcu <rcu@...r.kernel.org>, Josh Triplett <josh@...htriplett.org>,
        rostedt <rostedt@...dmis.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>
Subject: Re: tasks-trace RCU: question about grace period forward progress

On Thu, Feb 25, 2021 at 11:23:18PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 25, 2021 at 10:33:21AM -0800, Paul E. McKenney wrote:
> > One question for Peter...  Does each and every context switch imply a
> > full barrier?
> 
> Yes, also see the smp_mb__after_spinlock() in __schedule() :-)

Whew!!!  ;-)

Yeah, I could make RCU Tasks Trace deal with lack of a full barrier in
that case, but I would rather not...  I could imagine optimizing so
that the full barrier happened only when tasks migrated, but I could
also imagine a world of hurt stemming from such an optimization!

							Thanx, Paul