linux-kernel - Re: [PATCH v2 1/1] stop_machine: Remove stop_cpus_lock and lg_double

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Message-ID: <20151123215339.GF19072@mtj.duckdns.org>
Date:	Mon, 23 Nov 2015 16:53:39 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] stop_machine: Remove stop_cpus_lock and
 lg_double_lock/unlock()

Hello, Oleg.

On Sat, Nov 21, 2015 at 07:11:48PM +0100, Oleg Nesterov wrote:
> stop_two_cpus() and stop_cpus() use stop_cpus_lock to avoid the deadlock,
> we need to ensure that the stopper functions can't be queued "backwards"
> from one another. This doesn't look nice; if we use lglock then we do not
> really need stopper->lock, cpu_stop_queue_work() could use lg_local_lock()
> under local_irq_save().

Yeah, removing stopper->lock would be nice.

> OTOH it would be even better to avoid lglock in stop_machine.c and remove
> lg_double_lock(). This patch adds "bool stop_cpus_in_progress" set/cleared
> by queue_stop_cpus_work(), and changes cpu_stop_queue_two_works() to busy
> wait until it is cleared.
> 
> queue_stop_cpus_work() sets stop_cpus_in_progress = T lockless, but after
> it queues a work on CPU1 it must be visible to stop_two_cpus(CPU1, CPU2)
> which checks it under the same lock. And since stop_two_cpus() holds the
> 2nd lock too, queue_stop_cpus_work() can not clear stop_cpus_in_progress
> if it is also going to queue a work on CPU2, it needs to take that 2nd
> lock to do this.

Isn't this a lot more subtler than the other direction?  Unless
there's a clear performance advantage to removing stopper->lock, using
lglock for both stop_two and stop_machine seems like an
easier-to-follow approach to me.

Thanks.

-- 
tejun
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