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Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to
 monitor lockless pgtable walks
To: Peter Zijlstra <peterz@infradead.org>, Leonardo Bras
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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