linux-kernel - Re: [PATCH] rcu: use try_cmpxchg in check_cpu

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Date:   Tue, 28 Feb 2023 13:56:54 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Uros Bizjak <ubizjak@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

On Tue, Feb 28, 2023 at 04:41:24PM -0500, Steven Rostedt wrote:
> On Tue, 28 Feb 2023 13:29:11 -0800
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > All well and good, but the stall-warning code is nowhere near a fastpath.
> > 
> > Is try_cmpxchg() considered more readable in this context?
> 
> 
> -	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
> +	    try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
> 
> It's basically the same :-/

That was my assessment.  ;-)

> But looking at this use case, I'd actually NAK it, as it is misleading.
> 
> As try_cmpxchg() is used to get rid of the updating of the old value. As in
> the ring buffer code we had:
> 
> void ring_buffer_record_off(struct trace_buffer *buffer)
> {
> 	unsigned int rd;
> 	unsigned int new_rd;
> 
> 	do {
> 		rd = atomic_read(&buffer->record_disabled);
> 		new_rd = rd | RB_BUFFER_OFF;
> 	} while (!atomic_cmpxchg(&buffer->record_disabled, &rd, new_rd) != rd);
> }
> 
> and the try_cmpxchg() converted it to:
> 
> void ring_buffer_record_off(struct trace_buffer *buffer)
> {
> 	unsigned int rd;
> 	unsigned int new_rd;
> 
> 	rd = atomic_read(&buffer->record_disabled);
> 	do {
> 		new_rd = rd | RB_BUFFER_OFF;
> 	} while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));
> }
> 
> Which got rid of the need to constantly update the rd variable (cmpxchg
> will load rax with the value read, so it removes the need for an extra
> move).
> 
> But in your case, we don't need to update js, in which case the
> try_cmpxchg() does.
> 
> The patch that Uros sent me for the ring buffer code also does some of
> that, which I feel is wrong.
> 
> So with that, I would nack the patch.

OK, I will leave this one out.

							Thanx, Paul