From mboxrd@z Thu Jan  1 00:00:00 1970
From: John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to
 monitor lockless pgtable walks
Date: Thu, 3 Oct 2019 13:40:38 -0700
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To: Peter Zijlstra <peterz@infradead.org>, Leonardo Bras <leonardo@linux.ibm.com>
Cc: Song Liu <songliubraving@fb.com>, Michal Hocko <mhocko@suse.com>, "Dmitry V. Levin" <ldv@altlinux.org>, Keith Busch <keith.busch@intel.com>, linux-mm@kvack.org, Paul Mackerras <paulus@samba.org>, Christoph Lameter <cl@linux.com>, Ira Weiny <ira.weiny@intel.com>, Dan Williams <dan.j.williams@intel.com>, Elena Reshetova <elena.reshetova@intel.com>, linux-arch@vger.kernel.org, Santosh Sivaraj <santosh@fossix.org>, Davidlohr Bueso <dave@stgolabs.net>, "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>, Mike Rapoport <rppt@linux.ibm.com>, Jason Gunthorpe <jgg@ziepe.ca>, Vlastimil Babka <vbabka@suse.cz>, Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Alexey Dobriyan <adobriyan@gmail.com>, Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>, Ralph Campbell <rcampbel>
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On 10/3/19 4:51 AM, Peter Zijlstra wrote:
> On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote:
>> On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote:
...
> 
> I'm still really confused about this barrier. It just doesn't make
> sense.
> 
> If an interrupt happens before the local_irq_disable()/save(), then it
> will discard any and all speculation that would be in progress to handle
> the exception.
> 

Hi Peter,

So, would that imply that it's correct to apply approximately the following
patch:

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 1adbb8a371c7..cf41eff37e24 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -2099,9 +2099,9 @@ INTERRUPT DISABLING FUNCTIONS
 -----------------------------
 
 Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
-(RELEASE equivalent) will act as compiler barriers only.  So if memory or I/O
-barriers are required in such a situation, they must be provided from some
-other means.
+(RELEASE equivalent) will act as full memory barriers. This is because, for
+all supported CPU architectures, interrupt arrival causes all speculative
+memory accesses to be discarded.
 
?

We're also discussing this over in [1] ("mm: don't expose non-hugetlb page to
fast gup prematurely"), so I'm adding Paul to this thread here as well.

[1] https://lore.kernel.org/r/20191002092447.GC9320@quack2.suse.cz

thanks,
-- 
John Hubbard
NVIDIA