linux-kernel - Re: [PATCH V2 9/9] jump_label: Batch up if arch supports it

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Message-ID: <20181218153208.6f8bd858@gandalf.local.home>
Date:   Tue, 18 Dec 2018 15:32:08 -0500
From:   Steven Rostedt <rostedt@...dmis.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>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Zhou Chengming <zhouchengming1@...wei.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        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 V2 9/9] jump_label: Batch up if arch supports it

On Tue, 18 Dec 2018 20:33:48 +0100
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> >> +#else
> >> +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_check(entry, init))
> >> +			continue;
> >> +
> >> +		if (arch_jump_label_transform_queue(entry,
> >> +						    jump_label_type(entry)))
> >> +			continue;
> >> +
> >> +		/*
> >> +		 * Queue's overflow: Apply the current queue, and then
> >> +		 * queue again. If it stills not possible to queue, BUG!
> >> +		 */
> >> +		arch_jump_label_transform_apply();
> >> +		if (!arch_jump_label_transform_queue(entry,
> >> +						     jump_label_type(entry))) {
> >> +			BUG();  
> > 
> > Why BUG()? Do you really want to crash Linus's machine?  
> 
> I am using BUG() because that is what I see in other part of jump_label code:
> 	If something goes wrong:
> 		BUG().

Where? Mostly we have BUILD_BUG_ON() which isn't bad at all.

The only other BUG I see in the jump label code is in the arch specific
code and that's from 2012. Lately, we are trying to get rid of BUG()
and panic() entirely, with a few exceptions (where there's really no
way to return. Like when the function graph stack is corrupted, and we
don't know where to go).

If there's a way to continue in a critical state, it's best to WARN()
and continue on. That way, the user can have a chance to see what
happened.

> 
> What I could do here is:
> 
> Add a "fallback" boll that is disabled by default.
> If I hit this case:
> 	WARN()
> 	turn "fallback" on, returning to the old mode (without batch)
> 
> Sound better?

Yes, please do.

Thanks!

-- Steve