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

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Message-ID: <20121002214327.GA29218@linux.vnet.ibm.com>
Date:	Tue, 2 Oct 2012 14:43:27 -0700
From:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alexander Graf <agraf@...e.de>, 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

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? Does it go away by reverting
that commit against mainline? Just trying to narrow down my focus.

Thanks,
Nish

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