linux-kernel - Re: [PATCH 12/35] workqueue: update cwq alignement

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Date:	Tue, 29 Jun 2010 09:39:41 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk,
	dwalker@...eaurora.org, stefanr@...6.in-berlin.de,
	florian@...kler.org, andi@...stfloor.org, mst@...hat.com,
	randy.dunlap@...cle.com
Subject: Re: [PATCH 12/35] workqueue: update cwq alignement

Hello,

On 06/29/2010 12:47 AM, Frederic Weisbecker wrote:
>>  void __init init_workqueues(void)
>>  {
>> +	/*
>> +	 * cwqs are forced aligned according to WORK_STRUCT_FLAG_BITS.
>> +	 * Make sure that the alignment isn't lower than that of
>> +	 * unsigned long long.
>> +	 */
>> +	BUILD_BUG_ON(__alignof__(struct cpu_workqueue_struct) <
>> +		     __alignof__(unsigned long long));
>> +
> 
> But they are not allocated contiguously as we use the per cpu offsets.
> So why does the struct itself need to be aligned? Only the base pointer
> of its dynamic allocation needs to be aligned. Or am I missing something?

work->data doesn't store the percpu pointer but the address of cwq of
that specific cpu as returned by per_cpu_ptr(), so each element needs
to be aligned.  Besides, if the percpu ptr is aligned the elements are
aligned so they aren't different things.

> This is crashing my build in x86-32, unless I force an alignment to 8, or
> I just remove this build check.

Heh, how did that happen?  I'll investigate.  Can you please attach
your .config?

Thanks.

-- 
tejun
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