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

[Peter Zijlstra]

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.