linux-kernel - Re: [RFC PATCH 1/1] hpref: Hazard Pointers with Reference Counter

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Message-ID: <CAJhGHyBOwwBhCW96MyFGPVWed2ko37mLeHzta2BzL6T-TzUnmg@mail.gmail.com>
Date: Sun, 22 Sep 2024 05:07:23 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Boqun Feng <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, 
	linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Alan Stern <stern@...land.harvard.edu>, 
	John Stultz <jstultz@...gle.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Frederic Weisbecker <frederic@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Zqiang <qiang.zhang1211@...il.com>, 
	Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>, 
	Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com, Mateusz Guzik <mjguzik@...il.com>, 
	rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH 1/1] hpref: Hazard Pointers with Reference Counter

> +/*
> + * hpref_hp_get: Obtain a reference to a stable object, protected either
> + *               by hazard pointer (fast-path) or using reference
> + *               counter as fall-back.
> + */
> +static inline
> +bool hpref_hp_get(struct hpref_node **node_p, struct hpref_ctx *ctx)
> +{
> +       int cpu = rseq_current_cpu_raw();
> +       struct hpref_percpu_slots *cpu_slots = rseq_percpu_ptr(hpref_percpu_slots, cpu);
> +       struct hpref_slot *slot = &cpu_slots->slots[cpu_slots->current_slot];
> +       bool use_refcount = false;
> +       struct hpref_node *node, *node2;
> +       unsigned int next_slot;
> +
> +retry:
> +       node = uatomic_load(node_p, CMM_RELAXED);
> +       if (!node)
> +               return false;
> +       /* Use rseq to try setting current slot hp. Store B. */
> +       if (rseq_load_cbne_store__ptr(RSEQ_MO_RELAXED, RSEQ_PERCPU_CPU_ID,
> +                               (intptr_t *) &slot->node, (intptr_t) NULL,
> +                               (intptr_t) node, cpu)) {
> +               slot = &cpu_slots->slots[HPREF_EMERGENCY_SLOT];

Can @cpu be possibly changed? if it can, it seems @cpu and @cpu_slots
should be updated first.

> +               use_refcount = true;
> +               /*
> +                * This may busy-wait for another reader using the
> +                * emergency slot to transition to refcount.
> +                */
> +               caa_cpu_relax();
> +               goto retry;
> +       }