On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote:
> On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >
> > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler
> > > > errors when we add a const modifier to vma->vm_flags.
> > > >
> > > > ...
> > > >
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> > > > * orig->shared.rb may be modified concurrently, but the clone
> > > > * will be reinitialized.
> > > > */
> > > > - *new = data_race(*orig);
> > > > + memcpy(new, orig, sizeof(*new));
> > >
> > > The data_race() removal is unchangelogged?
> >
> > True. I'll add a note in the changelog about that. Ideally I would
> > like to preserve it but I could not find a way to do that.
>
> Perhaps Paul can comment?
>
> I wonder if KCSAN knows how to detect this race, given that it's now in
> a memcpy. I assume so.
I ran an experiment memcpy()ing between a static array and an onstack
array, and KCSAN did not complain. But maybe I was setting it up wrong.
This is what I did:
long myid = (long)arg; /* different value for each task */
static unsigned long z1[10] = { 0 };
unsigned long z2[10];
...
memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0]));
for (zi = 0; zi < ARRAY_SIZE(z1); zi++)
z2[zi] += myid;
memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0]));
Adding Marco on CC for his thoughts.
Thanx, Paul
