linux-kernel - Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 12 Sep 2016 15:05:38 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Cheng Chao <cs.os.kernel@...il.com>, mingo@...nel.org,
        tj@...nel.org, akpm@...ux-foundation.org, chris@...is-wilson.co.uk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful
        works for CONFIG_PREEMPT_NONE

On 09/12, Peter Zijlstra wrote:
>
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -121,6 +121,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
>  	cpu_stop_init_done(&done, 1);
>  	if (!cpu_stop_queue_work(cpu, &work))
>  		return -ENOENT;
> +	/*
> +	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
> +	 * by doing a preemption.
> +	 */
> +	cond_resched();

Yes, this is what I tried to suggest too.

But this leads to the question which I wanted to ask many times.

Why cond_resched() is not NOP if CONFIG_PREEMPT=y ?

Perhaps we have some users like, just for example,

	preempt_enable_no_resched();
	cond_resched();

which actually want the should_resched() check even if CONFIG_PREEMPT,
but most callers do not?

Oleg.