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

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?

> > 	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));
> > 	__this_cpu_write(unwind_ctx_ctr, ctr+1);
> > 	return cookie;
> 
> I was trying to avoid the overhead of the cmpxchg.

We're going to be doing userspace stack unwinding, I don't think
overhead is a real concern.

> 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
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.