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

[Josh Poimboeuf]

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