linux-kernel - Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

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Date: Sat, 27 Jan 2024 23:54:46 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	John Stultz <jstultz@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Rik van Riel <riel@...riel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Giovanni Gherdovich <ggherdovich@...e.cz>,
	Lukasz Luba <lukasz.luba@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
> +static bool tmigr_inactive_up(struct tmigr_group *group,
> +			      struct tmigr_group *child,
> +			      void *ptr)
> +{
> +	union tmigr_state curstate, newstate, childstate;
> +	struct tmigr_walk *data = ptr;
> +	bool walk_done;
> +	u8 childmask;
> +
> +	childmask = data->childmask;
> +	curstate.state = atomic_read(&group->migr_state);
> +	childstate.state = 0;
> +
> +	do {

So I got the confirmation from Boqun (+Cc) and Paul that a failing cmpxchg
may not order the load of the old value against subsequent loads. And
that may apply to atomic_try_cmpxchg() as well.

Therefore you not only need to turn group->migr_state read into
an atomic_read_acquire() but you also need to do this on each iteration
of this loop. For example you can move the read_acquire right here.

Thanks.

> +		if (child)
> +			childstate.state = atomic_read(&child->migr_state);
> +
> +		newstate = curstate;
> +		walk_done = true;
> +
> +		/* Reset active bit when the child is no longer active */
> +		if (!childstate.active)
> +			newstate.active &= ~childmask;
> +
> +		if (newstate.migrator == childmask) {
> +			/*
> +			 * Find a new migrator for the group, because the child
> +			 * group is idle!
> +			 */
> +			if (!childstate.active) {
> +				unsigned long new_migr_bit, active = newstate.active;
> +
> +				new_migr_bit = find_first_bit(&active, BIT_CNT);
> +
> +				if (new_migr_bit != BIT_CNT) {
> +					newstate.migrator = BIT(new_migr_bit);
> +				} else {
> +					newstate.migrator = TMIGR_NONE;
> +
> +					/* Changes need to be propagated */
> +					walk_done = false;
> +				}
> +			}
> +		}
> +
> +		newstate.seq++;
> +
> +		WARN_ON_ONCE((newstate.migrator != TMIGR_NONE) && !(newstate.active));
> +
> +	} while (!atomic_try_cmpxchg(&group->migr_state, &curstate.state, newstate.state));