RE: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is not atomic

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2011-05-18 at 09:35 -0600, Moore, Eric wrote:
> I worked the original defect a couple months ago, and Kashyap is now
> getting around to posting my patch's.
> 
> This original defect has nothing to do with PPC64.  The original
> problem was only on x86.    It only became a problem on PPC64 when I
> tried to fix the original x86 issue by copying the writeq code from
> the linux headers, then it broke PPC64.   I doubt that broken patch
> was ever posted. Anyways, back to the original defect.  The reason it
> because a problem for x86 is because the kernel headers had a
> implementation of writeq in the arch/x86 headers, which means our
> internal implementation of writeq is not being used.  The writeq
> implementation in the kernel is total wrong for arch/x86 because it
> doesn't not have spin locks, and if two processor simultaneously doing
> two separate 32bit pci writes, then what is received by controller
> firmware is out of order.   This change occurs between Red Hat RHEL5
> and RHEL6.  In RHEL5, this writeq was not implemented in arch/x86
> headers, and our driver internal implementation of write was used.

You may also want to look at Milton's comments, it looks like the way
you do init_completion followed immediately by wait_completion is racy.

You should init the completion before you do the IO that will eventually
trigger complete() to be called.

Cheers,
Ben.