Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

Oleg Nesterov
2019-09-04T12:03:44Z

On 09/04, Peter Zijlstra wrote:
>
> + struct task_struct *g, *t;
> +
> + read_lock(&tasklist_lock);
> + do_each_thread(g, t) {

for_each_process_thread() looks better

> + if (t->mm == mm) {
> + atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED,
> + &t->membarrier_state);
> + }

then you also need to change dup_task_struct(), it should clear
->membarrier_state unless CLONE_VM.

And probably unuse_mm() should clear current->membarrier_state too.

Hmm. And it can race with copy_process() anyway, tasklist_lock can't
really help. So copy_process() needs to do

write_lock_irq(&tasklist_lock);
...

if (clone_flags & CLONE_VM)
p->membarrier_state = current->membarrier_state;
else
p->membarrier_state = 0;

Oleg.