linux-kernel - Re: [PATCH 40/45] powerpc, irq: Use GFP_ATOMIC allocations in atomic context

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Message-ID: <51C9ED61.1060306@linux.vnet.ibm.com>
Date:	Wed, 26 Jun 2013 00:50:01 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Michael Ellerman <michael@...erman.id.au>, tglx@...utronix.de,
	peterz@...radead.org, tj@...nel.org, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org,
	laijs@...fujitsu.com, rostedt@...dmis.org,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	sbw@....edu, fweisbec@...il.com, zhong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Ian Munsie <imunsie@....ibm.com>
Subject: Re: [PATCH 40/45] powerpc, irq: Use GFP_ATOMIC allocations in atomic
 context

On 06/25/2013 08:43 AM, Benjamin Herrenschmidt wrote:
> On Tue, 2013-06-25 at 12:58 +1000, Michael Ellerman wrote:
>> On Tue, Jun 25, 2013 at 12:13:04PM +1000, Benjamin Herrenschmidt wrote:
>>> On Tue, 2013-06-25 at 12:08 +1000, Michael Ellerman wrote:
>>>> We're not checking for allocation failure, which we should be.
>>>>
>>>> But this code is only used on powermac and 85xx, so it should probably
>>>> just be a TODO to fix this up to handle the failure.
>>>
>>> And what can we do if they fail ?
>>
>> Fail up the chain and not unplug the CPU presumably.
> 
> BTW. Isn't Srivatsa series removing the need to stop_machine() for
> unplug ? 

Yes.

That should mean we should be able to use GFP_KERNEL no ?

No, because whatever code was being executed in stop_machine() context
would still be executed with interrupts disabled. So allocations that
can sleep would continue to be forbidden in this path.

In the CPU unplug sequence, the CPU_DYING notifications (and the surrounding
code) is guaranteed to be run:
a. _on_ the CPU going offline
b. with interrupts disabled on that CPU.

My patchset will retain these guarantees even after removing stop_machine().
And these are required for the correct execution of the code in this path,
since they rely on these semantics.

So I guess I'll retain the patch as it is. Thank you!

Regards,
Srivatsa S. Bhat

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