Re: Are jump labels broken on 6.11-rc1?

Thomas Gleixner
2024-08-07T14:03:14Z

On Tue, Aug 06 2024 at 12:38, Peter Zijlstra wrote:
> On Tue, Aug 06, 2024 at 11:44:13AM +0200, Peter Zijlstra wrote:
> I've ended up with this, not exactly pretty :/
>
> -static bool static_key_slow_try_dec(struct static_key *key)
> +static bool static_key_dec(struct static_key *key, bool fast)
> {
> int v;
>
> @@ -268,31 +268,45 @@ static bool static_key_slow_try_dec(struct static_key *key)
> v = atomic_read(&key->enabled);
> do {
> /*
> - * Warn about the '-1' case though; since that means a
> - * decrement is concurrent with a first (0->1) increment. IOW
> - * people are trying to disable something that wasn't yet fully
> - * enabled. This suggests an ordering problem on the user side.
> + * Warn about the '-1' case; since that means a decrement is
> + * concurrent with a first (0->1) increment. IOW people are
> + * trying to disable something that wasn't yet fully enabled.
> + * This suggests an ordering problem on the user side.
> + *
> + * Warn about the '0' case; simple underflow.
> + *
> + * Neither case should succeed and change things.

Which is confusing because the fastpath will drop down into the slowpath
due to this.

> + */
> + if (WARN_ON_ONCE(v <= 0))
> + return false;

This forces the fastpath into the slowpath. I assume this on purpose to
handle the concurrent 'first enable (enabled == -1)'. But hell this is
not comprehensible without a comment.

> static void __static_key_slow_dec_cpuslocked(struct static_key *key)
> {
> lockdep_assert_cpus_held();
>
> - if (static_key_slow_try_dec(key))
> + if (static_key_dec(key, true)) // dec-not-one

Eeew.

Something like the below?

Thanks,

tglx
---
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -168,8 +168,8 @@ bool static_key_slow_inc_cpuslocked(stru
jump_label_update(key);
/*
* Ensure that when static_key_fast_inc_not_disabled() or
- * static_key_slow_try_dec() observe the positive value,
- * they must also observe all the text changes.
+ * static_key_dec() observe the positive value, they must also
+ * observe all the text changes.
*/
atomic_set_release(&key->enabled, 1);
} else {
@@ -250,49 +250,71 @@ void static_key_disable(struct static_ke
}
EXPORT_SYMBOL_GPL(static_key_disable);

-static bool static_key_slow_try_dec(struct static_key *key)
+static bool static_key_dec(struct static_key *key, bool dec_not_one)
{
- int v;
+ int v = atomic_read(&key->enabled);

- /*
- * Go into the slow path if key::enabled is less than or equal than
- * one. One is valid to shut down the key, anything less than one
- * is an imbalance, which is handled at the call site.
- *
- * That includes the special case of '-1' which is set in
- * static_key_slow_inc_cpuslocked(), but that's harmless as it is
- * fully serialized in the slow path below. By the time this task
- * acquires the jump label lock the value is back to one and the
- * retry under the lock must succeed.
- */
- v = atomic_read(&key->enabled);
do {
/*
- * Warn about the '-1' case though; since that means a
- * decrement is concurrent with a first (0->1) increment. IOW
- * people are trying to disable something that wasn't yet fully
- * enabled. This suggests an ordering problem on the user side.
+ * Warn about the '-1' case; since that means a decrement is
+ * concurrent with a first (0->1) increment. IOW people are
+ * trying to disable something that wasn't yet fully enabled.
+ * This suggests an ordering problem on the user side.
+ *
+ * Warn about the '0' case; simple underflow.
*/
- WARN_ON_ONCE(v < 0);
- if (v <= 1)
- return false;
+ if (WARN_ON_ONCE(v <= 0))
+ return v;
+
+ if (dec_not_one && v == 1)
+ return v;
+
} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));

- return true;
+ return v;
+}
+
+/*
+ * Fastpath: Decrement if the reference count is greater than one
+ *
+ * Returns false, if the reference count is 1 or -1 to force the caller
+ * into the slowpath.
+ *
+ * The -1 case is to handle a decrement during a concurrent first enable,
+ * which sets the count to -1 in static_key_slow_inc_cpuslocked(). As the
+ * slow path is serialized the caller will observe 1 once it acquired the
+ * jump_label_mutex, so the slow path can succeed.
+ */
+static bool static_key_dec_not_one(struct static_key *key)
+{
+ int v = static_key_dec(key, true);
+
+ return v != 1 && v != -1;
+}
+
+/*
+ * Slowpath: Decrement and test whether the refcount hit 0.
+ *
+ * Returns true if the refcount hit zero, i.e. the previous value was one.
+ */
+static bool static_key_dec_and_test(struct static_key *key)
+{
+ int v = static_key_dec(key, false);
+
+ lockdep_assert_held(&jump_label_mutex);
+ return v == 1;
}

static void __static_key_slow_dec_cpuslocked(struct static_key *key)
{
lockdep_assert_cpus_held();

- if (static_key_slow_try_dec(key))
+ if (static_key_dec_not_one(key))
return;

guard(mutex)(&jump_label_mutex);
- if (atomic_cmpxchg(&key->enabled, 1, 0) == 1)
+ if (static_key_dec_and_test(key))
jump_label_update(key);
- else
- WARN_ON_ONCE(!static_key_slow_try_dec(key));
}

static void __static_key_slow_dec(struct static_key *key)
@@ -329,7 +351,7 @@ void __static_key_slow_dec_deferred(stru
{
STATIC_KEY_CHECK_USE(key);

- if (static_key_slow_try_dec(key))
+ if (static_key_dec_not_one(key))
return;

schedule_delayed_work(work, timeout);