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

From: Nishanth Aravamudan <[email-masked]>
Date: Tue, 2 Oct 2012 14:43:27 -0700

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