linux-next: missing merge fix for the access_once tree

linux-next: missing merge fix for the access_once tree

[Stephen Rothwell]

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Hi Linus,

I have been carrying this merge fix patch for some time.  It should
have gone into the merge of the access_once tree.

From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Sun, 7 Dec 2014 22:01:59 +0100
Subject: [PATCH] x86/xen/p2m: Replace ACCESS_ONCE with READ_ONCE

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

Change the p2m code to replace ACCESS_ONCE with READ_ONCE.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 arch/x86/xen/p2m.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 8b5db51..08ddf91 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -554,7 +554,7 @@ static bool alloc_p2m(unsigned long pfn)
 		mid_mfn = NULL;
 	}
 
-	p2m_pfn = pte_pfn(ACCESS_ONCE(*ptep));
+	p2m_pfn = pte_pfn(READ_ONCE(*ptep));
 	if (p2m_pfn == PFN_DOWN(__pa(p2m_identity)) ||
 	    p2m_pfn == PFN_DOWN(__pa(p2m_missing))) {
 		/* p2m leaf page is missing */
-- 
1.8.5.5

-- 
Cheers,
Stephen Rothwell                    sfr@canb.auug.org.au

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Re: linux-next: missing merge fix for the access_once tree

[Linus Torvalds]

On Mon, Dec 22, 2014 at 6:17 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>
> I have been carrying this merge fix patch for some time.  It should
> have gone into the merge of the access_once tree.

No, you had a different tree that got merged into linux-next, that
made it an error to do ACCESS_ONCE() on a structure.

The thing I merged didn't actually have that final commit, exactly so
that things wouldn't break from missing conversions.

So that patch isn't wrong, but it also isn't exactly a required fix
until the next merge window..

                      Linus

Re: linux-next: missing merge fix for the access_once tree

[Stephen Rothwell]

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Hi Linus,

On Mon, 22 Dec 2014 20:12:22 -0800 Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> On Mon, Dec 22, 2014 at 6:17 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> >
> > I have been carrying this merge fix patch for some time.  It should
> > have gone into the merge of the access_once tree.
> 
> No, you had a different tree that got merged into linux-next, that
> made it an error to do ACCESS_ONCE() on a structure.
> 
> The thing I merged didn't actually have that final commit, exactly so
> that things wouldn't break from missing conversions.
> 
> So that patch isn't wrong, but it also isn't exactly a required fix
> until the next merge window..

True, so "should" was a bit strong.

-- 
Cheers,
Stephen Rothwell                    sfr@canb.auug.org.au

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Re: linux-next: missing merge fix for the access_once tree

[Christian Borntraeger]

Am 23.12.2014 um 07:34 schrieb Stephen Rothwell:
> Hi Linus,
> 
> On Mon, 22 Dec 2014 20:12:22 -0800 Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>> On Mon, Dec 22, 2014 at 6:17 PM, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>>>
>>> I have been carrying this merge fix patch for some time.  It should
>>> have gone into the merge of the access_once tree.
>>
>> No, you had a different tree that got merged into linux-next, that
>> made it an error to do ACCESS_ONCE() on a structure.
>>
>> The thing I merged didn't actually have that final commit, exactly so
>> that things wouldn't break from missing conversions.
>>
>> So that patch isn't wrong, but it also isn't exactly a required fix
>> until the next merge window..
> 
> True, so "should" was a bit strong.
> 

I will re-add this up soon for my linux-next branch. 
There are some additional places which need to be changed, so give me some more days during the holidays

Christian