Re: [PATCH v4 07/11] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses

[Peter Zijlstra]

On Fri, Apr 24, 2020 at 06:11:35PM +0100, Will Deacon wrote:
> My apologies, you're completely right. I thought that PAE mandated 64-bit
> atomicity, like it does on 32-bit ARM, but that's apparently not the case
> and looking at the 32-bit x86 pgtable code they have to be really careful
> there.

They added CMPXCHG8B for PAE, but then we never used it for that because
it's dead slow (MS did use it I think). Instead we play horrible split
word games, grep for CONFIG_GUP_GET_PTE_LOW_HIGH.