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.