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

Josh Poimboeuf
2025-01-22T22:49:05Z

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.

> 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.

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.

--
Josh