[PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks

[Peter Zijlstra]

On Thu, Oct 03, 2019 at 01:40:38PM -0700, John Hubbard wrote:
> 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.
>  
> ?

No, you're misunderstanding. They imply nothing of the sort.