Merge tag 'trace-v4.17-rc5-vsprintf' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull memory barrier for from Steven Rostedt:
"The memory barrier usage in updating the random ptr hash for %p in
vsprintf is incorrect.

Instead of adding the read memory barrier into vsprintf() which will
cause a slight degradation to a commonly used function in the kernel
just to solve a very unlikely race condition that can only happen at
boot up, change the code from using a variable branch to a
static_branch.

Not only does this solve the race condition, it actually will improve
the performance of vsprintf() by removing the conditional branch that
is only needed at boot"

* tag 'trace-v4.17-rc5-vsprintf' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
vsprintf: Replace memory barrier with static_key for random_ptr_key update

lib/vsprintf.c

···
 	return number(buf, end, (unsigned long int)ptr, spec);
 }
 
-static bool have_filled_random_ptr_key __read_mostly;
+static DEFINE_STATIC_KEY_TRUE(not_filled_random_ptr_key);
 static siphash_key_t ptr_key __read_mostly;
+
+static void enable_ptr_key_workfn(struct work_struct *work)
+{
+	get_random_bytes(&ptr_key, sizeof(ptr_key));
+	/* Needs to run from preemptible context */
+	static_branch_disable(&not_filled_random_ptr_key);
+}
+
+static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);
 
 static void fill_random_ptr_key(struct random_ready_callback *unused)
 {
-	get_random_bytes(&ptr_key, sizeof(ptr_key));
-	/*
-	 * have_filled_random_ptr_key==true is dependent on get_random_bytes().
-	 * ptr_to_id() needs to see have_filled_random_ptr_key==true
-	 * after get_random_bytes() returns.
-	 */
-	smp_mb();
-	WRITE_ONCE(have_filled_random_ptr_key, true);
+	/* This may be in an interrupt handler. */
+	queue_work(system_unbound_wq, &enable_ptr_key_work);
 }
 
 static struct random_ready_callback random_ready = {
···
 	if (!ret) {
 		return 0;
 	} else if (ret == -EALREADY) {
-		fill_random_ptr_key(&random_ready);
+		/* This is in preemptible context */
+		enable_ptr_key_workfn(&enable_ptr_key_work);
 		return 0;
 	}
 
···
 	unsigned long hashval;
 	const int default_width = 2 * sizeof(ptr);
 
-	if (unlikely(!have_filled_random_ptr_key)) {
+	if (static_branch_unlikely(&not_filled_random_ptr_key)) {
 		spec.field_width = default_width;
 		/* string length must be less than default_width */
 		return string(buf, end, "(ptrval)", spec);