linux-kernel - Re: [PATCH 4/8] shazptr: Avoid synchronize

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Message-ID: <20250625114037.GD1613376@noisy.programming.kicks-ass.net>
Date: Wed, 25 Jun 2025 13:40:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, lkmm@...ts.linux.dev,
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, Breno Leitao <leitao@...ian.org>,
	aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com,
	jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>
Subject: Re: [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting

On Tue, Jun 24, 2025 at 08:10:57PM -0700, Boqun Feng wrote:

> +/* Scan structure for synchronize_shazptr(). */
> +struct shazptr_scan {
> +	/* The scan kthread */
> +	struct task_struct *thread;
> +
> +	/* Wait queue for the scan kthread */
> +	struct swait_queue_head wq;
> +
> +	/* Whether the scan kthread has been scheduled to scan */
> +	bool scheduled;
> +
> +	/* The lock protecting ->queued and ->scheduled */
> +	struct mutex lock;
> +
> +	/* List of queued synchronize_shazptr() request. */
> +	struct list_head queued;
> +
> +	int cpu_grp_size;
> +
> +	/* List of scanning synchronize_shazptr() request. */
> +	struct list_head scanning;
> +
> +	/* Buffer used for hazptr slot scan, nr_cpu_ids slots*/
> +	struct shazptr_snapshot* snaps;
> +};

I find this style very hard to read, also the order of things is weird.

struct shazptr_scan {
	struct task_struct	*thread;
	struct swait_queue_head wq;
	struct list_head	scanning;

	struct mutex		lock;
	struct list_head	queued;    /* __guarded_by(lock) */
	bool			scheduled; /* __guarded_by(lock) */

	struct shazptr_snapshot snaps[0] __counted_by(nr_cpu_ids);
};

(the __guarded_by() thing will come with Thread-Safety support that
Google is still cooking in clang)

> +static struct shazptr_scan shazptr_scan;

And then make this a pointer, and allocate the whole thing as a single
data structure with however much snaps data you need.