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

[Peter Zijlstra]

On Tue, Jan 21, 2025 at 06:31:22PM -0800, Josh Poimboeuf wrote:

> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> index 2f38055cce48..939c94abaa50 100644
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -29,27 +29,49 @@ static u64 ctx_to_cookie(u64 cpu, u64 ctx)
>  
>  /*
>   * Read the task context cookie, first initializing it if this is the first
> - * call to get_cookie() since the most recent entry from user.
> + * call to get_cookie() since the most recent entry from user.  This has to 
> be
> + * done carefully to coordinate with unwind_deferred_request_nmi().
>   */
>  static u64 get_cookie(struct unwind_task_info *info)
>  {
>       u64 ctx_ctr;
>       u64 cookie;
> -     u64 cpu;
>  
>       guard(irqsave)();
>  
> -     cookie = info->cookie;
> +     cookie = READ_ONCE(info->cookie);
>       if (cookie)
>               return cookie;
>  
> +     ctx_ctr = __this_cpu_read(unwind_ctx_ctr);
>  
> -     cpu = raw_smp_processor_id();
> -     ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);
> -     info->cookie = ctx_to_cookie(cpu, ctx_ctr);
> +     /* Read ctx_ctr before info->nmi_cookie */
> +     barrier();
> +
> +     cookie = READ_ONCE(info->nmi_cookie);
> +     if (cookie) {
> +             /*
> +              * This is the first call to get_cookie() since an NMI handler
> +              * first wrote it to info->nmi_cookie.  Sync it.
> +              */
> +             WRITE_ONCE(info->cookie, cookie);
> +             WRITE_ONCE(info->nmi_cookie, 0);
> +             return cookie;
> +     }
> +
> +     /*
> +      * Write info->cookie.  It's ok to race with an NMI here.  The value of
> +      * the cookie is based on ctx_ctr from before the NMI could have
> +      * incremented it.  The result will be the same even if cookie or
> +      * ctx_ctr end up getting written twice.
> +      */
> +     cookie = ctx_to_cookie(raw_smp_processor_id(), ctx_ctr + 1);
> +     WRITE_ONCE(info->cookie, cookie);
> +     WRITE_ONCE(info->nmi_cookie, 0);
> +     barrier();
> +     __this_cpu_write(unwind_ctx_ctr, ctx_ctr + 1);
>  
>       return cookie;
> -
>  }

Oh gawd. Can we please do something simple like:

        guard(irqsave)();
        cpu = raw_smp_processor_id();
        ctr = __this_cpu_read(unwind_ctx_cnt);
        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;