Re: [PATCH v2] mm/memory.c: do_fault: avoid usage of stale vm_area_struct

[Jan Stancek]

----- Original Message -----
> Hello Jan,
> 
> On Sat, Mar 02, 2019 at 07:19:39PM +0100, Jan Stancek wrote:
> > +	struct mm_struct *vm_mm = READ_ONCE(vma->vm_mm);
> 
> The vma->vm_mm cannot change under gcc there, so no need of
> READ_ONCE. The release of mmap_sem has release semantics so the
> vma->vm_mm access cannot be reordered after up_read(mmap_sem) either.
> 
> Other than the above detail:
> 
> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Thank you for review, I dropped READ_ONCE and sent v3 with your
Reviewed-by included. I also successfully re-ran tests over-night.

> Would this not need a corresponding WRITE_ONCE() in vma_init() ?

There's at least 2 context switches between, so I think it wouldn't matter.
My concern was gcc optimizing out vm_mm, and vma->vm_mm access happening only
after do_read_fault().