linux-kernel - Re: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu

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Date:	Mon, 01 Jun 2009 10:22:47 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, rusty@...tcorp.com.au,
	mingo@...e.hu, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug

Lai Jiangshan wrote:
> 
> down_read()/up_read() can be nested within down_read()/up_read(),
> so get_online_cpus() is recursive.
> 
> And thanks to cpu_hotplug.active_writer, get_online_cpus()/put_online_cpus()
> are allowd to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
> So cpu_hotplug_begin() DO NOT blocks readers who are in CPU notifiers.
> 

Lai Jiangshan wrote:
> 
> The current code drops mutex when get_online_cpus() succeeds, BUT it
> increases the counter as what down_read() does. I think the current
> code has the same deadlocks which the down_read()-implement has.
> 
> Since the current code use mutex + counter to implement a "down_read()",
> why not use the down_read() directly?
> And down_read() can be checked by lockdep.
> 

Ouch, the kernel rw_semaphore is not Read-preference. All what I said
is garbage. I did miss this, sorry for bothered you all.

Lai

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