linux-kernel - Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Message-ID: <50C2366C.3000204@linux.vnet.ibm.com>
Date:	Sat, 08 Dec 2012 00:03:16 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
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, oleg@...hat.com, 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 v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline
 from atomic context

On 12/07/2012 11:46 PM, Tejun Heo wrote:
> Hello, again.
> 
> On Fri, Dec 07, 2012 at 09:57:24AM -0800, Tejun Heo wrote:
>> possible.  Also, I think the right approach would be auditing each
>> get_online_cpus_atomic() callsites and figure out proper locking order
>> rather than implementing a construct this unusual especially as
>> hunting down the incorrect cases shouldn't be difficult given proper
>> lockdep annotation.
> 
> On the second look, it looks like you're implementing proper
> percpu_rwlock semantics

Ah, nice! I didn't realize that I was actually doing what I intended
to avoid! ;-)

Looking at the implementation of lglocks, and going by Oleg's earlier
comment that we just need to replace spinlock_t with rwlock_t in them
to get percpu_rwlocks, I was horrified at the kinds of circular locking
dependencies that they would be prone to, unless used carefully.

So I devised this scheme to be safe, while still having relaxed rules.
But if *this* is what percpu_rwlocks should ideally look like,
then great! :-)

> as readers aren't supposed to induce circular
> dependency directly.

Yep, in this scheme, nobody will end up in circular dependency.

>  Can you please work with Oleg to implement
> proper percpu-rwlock and use that for CPU hotplug rather than
> implementing it inside CPU hotplug?
> 

Sure, I'd be more than happy to!
 
Regards,
Srivatsa S. Bhat

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