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:
> +static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *cookie)
> +{
> +	struct unwind_task_info *info = &current->unwind_info;
> +	bool inited_cookie = false;
> +	int ret;
> +
> +	*cookie = info->cookie;
> +	if (!*cookie) {
> +		/*
> +		 * This is the first unwind request since the most recent entry
> +		 * from user.  Initialize the task cookie.
> +		 *
> +		 * Don't write to info->cookie directly, otherwise it may get
> +		 * cleared if the NMI occurred in the kernel during early entry
> +		 * or late exit before the task work gets to run.  Instead, use
> +		 * info->nmi_cookie which gets synced later by get_cookie().
> +		 */
> +		if (!info->nmi_cookie) {
> +			u64 cpu = raw_smp_processor_id();
> +			u64 ctx_ctr;
> +
> +			ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);

__this_cpu_inc_return() is *NOT* NMI safe IIRC.