linux-kernel - Re: [REGRESSION] nfsd crashing with 3.6.0-rc7 on PowerPC

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Message-Id: <E141CA01-F58C-47B1-8ED5-A314D1DEC968@suse.de>
Date:	Wed, 3 Oct 2012 00:31:09 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-nfs@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML List <linux-kernel@...r.kernel.org>,
	"J. Bruce Fields" <bfields@...ldses.org>, anton@...ba.org,
	skinsbursky@...allels.com, bfields@...hat.com,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [REGRESSION] nfsd crashing with 3.6.0-rc7 on PowerPC


On 03.10.2012, at 00:17, Nishanth Aravamudan wrote:

> On 02.10.2012 [23:47:39 +0200], Alexander Graf wrote:
>> 
>> On 02.10.2012, at 23:43, Nishanth Aravamudan wrote:
>> 
>>> Hi Ben,
>>> 
>>> On 02.10.2012 [10:58:29 +1000], Benjamin Herrenschmidt wrote:
>>>> On Mon, 2012-10-01 at 16:03 +0200, Alexander Graf wrote:
>>>>> Phew. Here we go :). It looks to be more of a PPC specific problem
>>>>> than it appeared as at first:
>>>> 
>>>> Ok, so I suspect the problem is the pushing down of the locks which
>>>> breaks with iommu backends that have a separate flush callback. In
>>>> that case, the flush moves out of the allocator lock.
>>>> 
>>>> Now we do call flush before we return, still, but it becomes racy
>>>> I suspect, but somebody needs to give it a closer look. I'm hoping
>>>> Anton or Nish will later today.
>>> 
>>> Started looking into this. If your suspicion were accurate, wouldn't the
>>> bisection have stopped at 0e4bc95d87394364f408627067238453830bdbf3
>>> ("powerpc/iommu: Reduce spinlock coverage in iommu_alloc and
>>> iommu_free")?
>>> 
>>> Alex, the error is reproducible, right?
>> 
>> Yes. I'm having a hard time to figure out if the reason my U4 based G5
>> Mac crashes and fails reading data is the same since I don't have a
>> serial connection there, but I assume so.
> 
> Ok, great, thanks. Yeah, that would imply (I think) that the I would
> have thought the lock pushdown in the above commit (or even in one of
> the others in Anton's series) would have been the real source if it was
> a lock-based race. But that's just my first sniff at what Ben was
> suggesting. Still reading/understanding the code.
> 
>>> Does it go away by reverting
>>> that commit against mainline? Just trying to narrow down my focus.
>> 
>> The patch doesn't revert that easily. Mind to provide a revert patch
>> so I can try?
> 
> The following at least builds on defconfig here:

Yes. With that patch applied, things work for me again.


Alex

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