On Wed, Oct 06, 2021 at 12:29:32PM +0200, Petr Mladek wrote:
> On Wed 2021-10-06 11:04:26, Peter Zijlstra wrote:
> > So it needs to be something like:
> >
> >
> > CPU0 CPU1
> >
> > <user>
> >
> > if (context_tracking_set_cpu_work(task_cpu(), CT_WORK_KLP))
> >
> > <kernel-entry>
> > klp_update_patch_state klp_update_patch_state()
> >
> >
> > So that CPU0 and CPU1 race to complete klp_update_patch_state() *before*
> > any regular (!noinstr) code gets run.
>
> Grr, you are right. I thought that we migrated the task when entering
> kernel even before. But it seems that we do it only when leaving
> the kernel in exit_to_user_mode_loop().
Yep... :-)
> > Which then means it needs to look something like:
> >
> > noinstr void klp_update_patch_state(struct task_struct *task)
> > {
> > struct thread_info *ti = task_thread_info(task);
> >
> > preempt_disable_notrace();
> > if (arch_test_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags)) {
> > /*
> > * Order loads of TIF_PATCH_PENDING vs klp_target_state.
> > * See klp_init_transition().
> > */
> > smp_rmb();
> > task->patch_state = __READ_ONCE(klp_target_state);
> > /*
> > * Concurrent against self; must observe updated
> > * task->patch_state if !TIF_PATCH_PENDING.
> > */
> > smp_mb__before_atomic();
>
> IMHO, smp_wmb() should be enough. We are here only when this
> CPU set task->patch_state right above. So that CPU running
> this code should see the correct task->patch_state.
Yes, I think smp_wmb() and smp_mb__before_atomic() are NOPS for all the
same architectures, so that might indeed be a better choice.
> The read barrier is needed only when @task is entering kernel and
> does not see TIF_PATCH_PENDING. It is handled by smp_rmb() in
> the "else" branch below.
>
> It is possible that both CPUs see TIF_PATCH_PENDING and both
> set task->patch_state. But it should not cause any harm
> because they set the same value. Unless something really
> crazy happens with the internal CPU busses and caches.
Right, not our problem :-) Lots would be broken beyond repair in that
case.
> > arch_clear_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags);
> > } else {
> > /*
> > * Concurrent against self, see smp_mb__before_atomic()
> > * above.
> > */
> > smp_rmb();
>
> Yeah, this is the counter part against the above smp_wmb().
>
> > }
> > preempt_enable_notrace();
> > }
>
> Now, I am scared to increase my paranoia level and search for even more
> possible races. I feel overwhelmed at the moment ;-)
:-)
Anyway, I still need to figure out how to extract this context tracking
stuff from RCU and not make a giant mess of things, so until that
time....
