linux-kernel - Re: [PATCH v2 07/18] mm/ksw: add atomic watch on/off operations

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Message-ID: <20250904064659.GV4067720@noisy.programming.kicks-ass.net>
Date: Thu, 4 Sep 2025 08:46:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jinchao Wang <wangjinchao600@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	"Naveen N . Rao" <naveen@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, linux-perf-users@...r.kernel.org,
	linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/18] mm/ksw: add atomic watch on/off operations

On Thu, Sep 04, 2025 at 08:21:04AM +0800, Jinchao Wang wrote:

> +static DEFINE_PER_CPU(call_single_data_t,
> +		      watch_csd) = CSD_INIT(ksw_watch_on_local_cpu,
> +					    &watch_info);
> +

> +int ksw_watch_on(u64 watch_addr, u64 watch_len)
> +{
> +	unsigned long flags;
> +	int cpu;
> +
> +	if (!watch_addr) {
> +		pr_err("KSW: watch with invalid address\n");
> +		return -EINVAL;
> +	}
> +
> +	spin_lock_irqsave(&watch_lock, flags);
> +
> +	/*
> +	 * check if already watched
> +	 */
> +	if (watch_info.addr != 0 && // not uninit
> +	    watch_info.addr != (unsigned long)&watch_holder && // installed
> +	    watch_addr != (unsigned long)&watch_holder) { //not restore
> +		spin_unlock_irqrestore(&watch_lock, flags);
> +		return -EBUSY;
> +	}
> +
> +	watch_info.addr = watch_addr;
> +	watch_info.len = watch_len;
> +
> +	spin_unlock_irqrestore(&watch_lock, flags);
> +
> +	if (watch_addr == (unsigned long)&watch_holder)
> +		pr_debug("KSW: watch off starting\n");
> +	else
> +		pr_debug("KSW: watch on starting\n");
> +
> +	for_each_online_cpu(cpu) {
> +		if (cpu == raw_smp_processor_id()) {
> +			ksw_watch_on_local_cpu(&watch_info);
> +		} else {
> +			call_single_data_t *csd = &per_cpu(watch_csd, cpu);
> +
> +			smp_call_function_single_async(cpu, csd);
> +		}
> +	}
> +
> +	return 0;
> +}

What do you think happens when two ksw_watch_on() instances run
concurrently?

What happens if a CPU comes online/offline concurrently with
ksw_watch_on()?