[+ Timur] On Thu, Sep 28, 2017 at 03:38:00PM -0400, Jon Masters wrote: > On 09/27/2017 11:49 AM, Will Deacon wrote: > > > The moral of the story is that read-after-read (same address) ordering *only* > > applies if READ_ONCE is used consistently. This means we need to fix page > > table dereferences in the core code as well as the arch code to avoid this > > problem. The two RFC patches in this series fix arm64 (which is a bigger fix > > that necessary since I clean things up too) and page_vma_mapped_walk. > > > > Comments welcome. > > Thanks for this Will. I'll echo Timur's comment that it would be ideal > to split this up into the critical piece needed for ordering > access/update to the PMD in the face of a THP split and separately have > the cosmetic cleanups. Needless to say, we've got a bunch of people who > are tracking this one and tracking it ready for backport. We just got > THP re-enabled so I'm pretty keen that we not have to disable again. Yeah, of course. I already posted a point diff to Yury in the original thread: http://lists.infradead.org/pipermail/linux-arm-kernel/2017-September/533299.html so I'd like to queue that as an arm64 fix after we've worked out the general direction of the full fix. I also don't see why other architectures (including x86) can't be hit by this, so an alternative (completely untested) approach would just be to take patch 2 of this series. The full fix isn't just cosmetic; it's also addressing the wider problem of unannotated racing page table accesses outside of the specific failure case we've run into. Will
