linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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:	Wed, 12 Dec 2012 18:17:20 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU
	offline from atomic context

On 12/11, Srivatsa S. Bhat wrote:
>
> IOW, the hotplug readers just increment/decrement their per-cpu refcounts
> when no writer is active.

plus cli/sti ;) and increment/decrement are atomic.

At first glance looks correct to me, but I'll try to read it carefully
later.

A couple of minor nits,

> +static DEFINE_PER_CPU(bool, writer_signal);

Why it needs to be per-cpu? It can be global and __read_mostly to avoid
the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
into a single cacheline...

> +void get_online_cpus_atomic(void)
> +{
> +	unsigned long flags;
> +
> +	preempt_disable();
> +
> +	if (cpu_hotplug.active_writer == current)
> +		return;
> +
> +	local_irq_save(flags);

Yes... this is still needed, we are going to increment reader_percpu_refcnt
unconditionally and this makes reader_nested_percpu() == T.

But,

> +void put_online_cpus_atomic(void)
> +{
> +	unsigned long flags;
> +
> +	if (cpu_hotplug.active_writer == current)
> +		goto out;
> +
> +	local_irq_save(flags);
> +
> +	/*
> +	 * We never allow heterogeneous nesting of readers. So it is trivial
> +	 * to find out the kind of reader we are, and undo the operation
> +	 * done by our corresponding get_online_cpus_atomic().
> +	 */
> +	if (__this_cpu_read(reader_percpu_refcnt))
> +		__this_cpu_dec(reader_percpu_refcnt);
> +	else
> +		read_unlock(&hotplug_rwlock);
> +
> +	local_irq_restore(flags);
> +out:
> +	preempt_enable();
> +}

Do we really need local_irq_save/restore in put_ ?

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/