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Date:   Mon, 10 Feb 2020 10:07:45 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Ingo Molnar <mingo@redhat.com>,
        Richard Fontana <rfontana@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure
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On Mon, Feb 10, 2020 at 12:05:52PM -0500, Steven Rostedt wrote:
> On Mon, 10 Feb 2020 10:46:16 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Furthermore, using srcu would be detrimental, because of how it has
> > smp_mb() in the read side primitives.
> 
> I didn't realize that there was a full memory barrier in the srcu read
> side. Seems to me that itself is rational for reverting it. And also a
> big NAK for any suggestion to have any of the function tracing to use
> it as well (which comes up here and there).

Yeah, that was added some years back when people were complaining about
three synchronize_sched()'s worth of latency for synchronize_srcu().

I did prepare a patch about a year ago that would allow an srcu_struct
to be set up to not need the read-side smp_mb() calls, but this means
longer-latency grace periods (though nowhere near as long as those of
synchronize_rcu_tasks()) and also that the "fast SRCU" readers cannot
be used from idle or offline CPUs.

But an easy change if it is useful.

							Thanx, Paul