[tip: locking/urgent] jump_label: Fix static_key_slow_dec() yet again

tip-bot2 for Peter Zijlstra
2024-09-06T14:41:17Z

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID: de752774f38bb766941ed1bf910ba5a9f6cc6bf7
Gitweb: https://git.kernel.org/tip/de752774f38bb766941ed1bf910ba5a9f6cc6bf7
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 07 Aug 2024 16:03:12 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 06 Sep 2024 16:29:22 +02:00

jump_label: Fix static_key_slow_dec() yet again

While commit 83ab38ef0a0b ("jump_label: Fix concurrency issues in
static_key_slow_dec()") fixed one problem, it created yet another,
notably the following is now possible:

slow_dec
if (try_dec) // dec_not_one-ish, false
// enabled == 1
slow_inc
if (inc_not_disabled) // inc_not_zero-ish
// enabled == 2
return

guard((mutex)(&jump_label_mutex);
if (atomic_cmpxchg(1,0)==1) // false, we're 2

slow_dec
if (try-dec) // dec_not_one, true
// enabled == 1
return
else
try_dec() // dec_not_one, false
WARN

Close this by creating two distinct operations, one dec_not_one()-like
for the fast path and one dec_and_test()-like for the slow path. Both
also taking the magic -1 value into account.

Thomas provided the more readable version with comments on.

Fixes: 83ab38ef0a0b ("jump_label: Fix concurrency issues in static_key_slow_dec()")
Reported-by: "Darrick J. Wong" <djwong@kernel.org>
Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/875xsc4ehr.ffs@tglx
---
kernel/jump_label.c | 83 ++++++++++++++++++++++++++++----------------
1 file changed, 54 insertions(+), 29 deletions(-)

diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 6dc76b5..0881fd2 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -168,8 +168,8 @@ bool static_key_slow_inc_cpuslocked(struct static_key *key)
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 +250,74 @@ void static_key_disable(struct static_key *key)
}
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.
+ *
+ * Notably 0 (underflow) returns success such that it bails without doing
+ * anything.
+ */
+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 +354,7 @@ void __static_key_slow_dec_deferred(struct static_key *key,
{
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);