linux-kernel - Re: [PATCH V5 6/7] jump_label: Batch updates if arch supports it

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Date:   Mon, 15 Apr 2019 13:50:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Chris von Recklinghausen <crecklin@...hat.com>,
        Jason Baron <jbaron@...mai.com>, Scott Wood <swood@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Clark Williams <williams@...hat.com>, x86@...nel.org
Subject: Re: [PATCH V5 6/7] jump_label: Batch updates if arch supports it

On Mon, Apr 01, 2019 at 10:58:18AM +0200, Daniel Bristot de Oliveira wrote:
> +int fallback_batch __read_mostly;
> +static void __jump_label_update(struct static_key *key,
> +				struct jump_entry *entry,
> +				struct jump_entry *stop,
> +				bool init)
> +{
> +	for_each_label_entry(key, entry, stop) {
> +
> +		if (!jump_label_can_update(entry, init))
> +			continue;
> +
> +		if (unlikely(fallback_batch)) {
> +			arch_jump_label_transform(entry, jump_label_type(entry));
> +			continue;
> +		}
> +
> +		if (!arch_jump_label_transform_queue(entry, jump_label_type(entry)))
> +			continue;

That reads wrong; 'if we cannot queue, continue'

> +
> +		/*
> +		 * Queue's overflow: Apply the current queue, and then try to
> +		 * queue again. If it stills fail to queue, fallback to the
> +		 * non-batch mode!
> +		 */
> +		arch_jump_label_transform_apply();
> +
> +		if (arch_jump_label_transform_queue(entry, jump_label_type(entry))) {
> +			WARN(1, "jump_label: batch mode failure!\n");
> +			fallback_batch = 1;
> +			arch_jump_label_transform(entry, jump_label_type(entry));
> +		}
> +	}
> +	arch_jump_label_transform_apply();
> +}

Hurmph...

	for_each_whatever(entry, stop, key) {
		if (!jump_label_can_update(entry, init))
			continue;

		if (!arch_jump_label_transform_queue(entry)) {
			/* queue full, flush */
			arch_jump_label_transform_apply();
			BUG_ON(!arch_jump_label_transform_queue(entry));
		}
	}
	arch_jump_label_transform_apply();

Also, see next patch.