Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral requests NMI-safe

On Thu, Jan 23, 2025 at 09:40:26AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 22, 2025 at 02:49:02PM -0800, Josh Poimboeuf wrote:
> > On Wed, Jan 22, 2025 at 03:15:05PM +0100, Peter Zijlstra wrote:
> > > On Tue, Jan 21, 2025 at 06:31:22PM -0800, Josh Poimboeuf wrote:
> > > Oh gawd. Can we please do something simple like:
> > > 
> > > 	guard(irqsave)();
> > > 	cpu = raw_smp_processor_id();
> > > 	ctr = __this_cpu_read(unwind_ctx_cnt);
> > 
> > Don't you need a compiler barrier here?  __this_cpu_read() doesn't have
> > one.
> 
> What for?

Hm, I guess it's not needed for this one.

> > > 	cookie = READ_ONCE(current->unwind_info.cookie);
> > > 	do {
> > > 		if (cookie)
> > > 			return cookie;
> > > 		cookie = ctx_to_cookie(cpu, ctr+1);
> > > 	} while (!try_cmpxchg64(&current->unwind_info.cookie, &cookie, cookie));

Should not the 2nd argument be &zero?

> > > 	__this_cpu_write(unwind_ctx_ctr, ctr+1);
> > > 	return cookie;
> > But also, the nmi_cookie is still needed for the case where the NMI
> > arrives before info->cookie gets cleared by early entry-from-user.
> 
> So how about we clear cookie (and set nr_entries to -1) at

I think we could set nr_entries to 0 instead of -1?

> return-to-user, after we've done the work loop and have interrupts
> disabled until we hit userspace.
>
> Any NMI that hits there will have to cause another entry anyway.

But there's a cookie mismatch:

    // return-to-user: IRQs disabled
    <NMI>
	current->unwind_info.cookie = 0x1234
    </NMI>
    unwind_exit_to_user_mode()
	current->unwind_info.cookie = 0
    IRET
<IRQ>
    task_work()
        callback(@cookie=WRONG)

-- 
Josh