linux-kernel - Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

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Message-ID: <20241029134918.GA14555@noisy.programming.kicks-ass.net>
Date: Tue, 29 Oct 2024 14:49:18 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kerne.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Mon, Oct 28, 2024 at 02:47:38PM -0700, Josh Poimboeuf wrote:

> +static void unwind_user_task_work(struct callback_head *head)
> +{
...
> +	guard(rwsem_read)(&callbacks_rwsem);
> +
> +	for_each_set_bit(i, &pending, UNWIND_MAX_CALLBACKS) {
> +		if (callbacks[i])
> +			callbacks[i]->func(&trace, cookie, privs[i]);
> +	}

I'm fairly sure people will come with pitchforks for that read-lock
there. They scale like shit on big systems. Please use SRCU or somesuch.

> +}
> +
> +int unwind_user_register(struct unwind_callback *callback, unwind_callback_t func)
> +{
> +	scoped_guard(rwsem_write, &callbacks_rwsem) {
> +		for (int i = 0; i < UNWIND_MAX_CALLBACKS; i++) {
> +			if (!callbacks[i]) {
> +				callback->func = func;
> +				callback->idx = i;
> +				callbacks[i] = callback;
> +				return 0;
> +			}
> +		}
> +	}
> +
> +	callback->func = NULL;
> +	callback->idx = -1;
> +	return -ENOSPC;
> +}
> +
> +int unwind_user_unregister(struct unwind_callback *callback)
> +{
> +	if (callback->idx < 0)
> +		return -EINVAL;
> +
> +	scoped_guard(rwsem_write, &callbacks_rwsem)
> +		callbacks[callback->idx] = NULL;
> +
> +	callback->func = NULL;
> +	callback->idx = -1;
> +
> +	return 0;
> +}