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Message-ID: <171863923908.10875.16051115443404235815.tip-bot2@tip-bot2>
Date: Mon, 17 Jun 2024 15:47:19 -0000
From: "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Yue Sun <samsun1006219@...il.com>, Xingwei Lee <xrivendell7@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: locking/core] jump_label: Fix concurrency issues in
static_key_slow_dec()
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 83ab38ef0a0b2407d43af9575bb32333fdd74fb2
Gitweb: https://git.kernel.org/tip/83ab38ef0a0b2407d43af9575bb32333fdd74fb2
Author: Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Mon, 10 Jun 2024 14:46:36 +02:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 11 Jun 2024 11:25:23 +02:00
jump_label: Fix concurrency issues in static_key_slow_dec()
The commit which tried to fix the concurrency issues of concurrent
static_key_slow_inc() failed to fix the equivalent issues
vs. static_key_slow_dec():
CPU0 CPU1
static_key_slow_dec()
static_key_slow_try_dec()
key->enabled == 1
val = atomic_fetch_add_unless(&key->enabled, -1, 1);
if (val == 1)
return false;
jump_label_lock();
if (atomic_dec_and_test(&key->enabled)) {
--> key->enabled == 0
__jump_label_update()
static_key_slow_dec()
static_key_slow_try_dec()
key->enabled == 0
val = atomic_fetch_add_unless(&key->enabled, -1, 1);
--> key->enabled == -1 <- FAIL
There is another bug in that code, when there is a concurrent
static_key_slow_inc() which enables the key as that sets key->enabled to -1
so on the other CPU
val = atomic_fetch_add_unless(&key->enabled, -1, 1);
will succeed and decrement to -2, which is invalid.
Cure all of this by replacing the atomic_fetch_add_unless() with a
atomic_try_cmpxchg() loop similar to static_key_fast_inc_not_disabled().
[peterz: add WARN_ON_ONCE for the -1 race]
Fixes: 4c5ea0a9cd02 ("locking/static_key: Fix concurrent static_key_slow_inc()")
Reported-by: Yue Sun <samsun1006219@...il.com>
Reported-by: Xingwei Lee <xrivendell7@...il.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20240610124406.422897838@linutronix.de
---
kernel/jump_label.c | 45 ++++++++++++++++++++++++++++----------------
1 file changed, 29 insertions(+), 16 deletions(-)
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 3218fa5..1f05a19 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -131,7 +131,7 @@ bool static_key_fast_inc_not_disabled(struct static_key *key)
STATIC_KEY_CHECK_USE(key);
/*
* Negative key->enabled has a special meaning: it sends
- * static_key_slow_inc() down the slow path, and it is non-zero
+ * static_key_slow_inc/dec() down the slow path, and it is non-zero
* so it counts as "enabled" in jump_label_update(). Note that
* atomic_inc_unless_negative() checks >= 0, so roll our own.
*/
@@ -150,7 +150,7 @@ bool static_key_slow_inc_cpuslocked(struct static_key *key)
lockdep_assert_cpus_held();
/*
- * Careful if we get concurrent static_key_slow_inc() calls;
+ * Careful if we get concurrent static_key_slow_inc/dec() calls;
* later calls must wait for the first one to _finish_ the
* jump_label_update() process. At the same time, however,
* the jump_label_update() call below wants to see
@@ -247,20 +247,32 @@ EXPORT_SYMBOL_GPL(static_key_disable);
static bool static_key_slow_try_dec(struct static_key *key)
{
- int val;
-
- val = atomic_fetch_add_unless(&key->enabled, -1, 1);
- if (val == 1)
- return false;
+ int v;
/*
- * The negative count check is valid even when a negative
- * key->enabled is in use by static_key_slow_inc(); a
- * __static_key_slow_dec() before the first static_key_slow_inc()
- * returns is unbalanced, because all other static_key_slow_inc()
- * instances block while the update is in progress.
+ * 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.
*/
- WARN(val < 0, "jump label: negative count!\n");
+ 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_ON_ONCE(v < 0);
+ if (v <= 1)
+ return false;
+ } while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
+
return true;
}
@@ -271,10 +283,11 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key)
if (static_key_slow_try_dec(key))
return;
- jump_label_lock();
- if (atomic_dec_and_test(&key->enabled))
+ guard(mutex)(&jump_label_mutex);
+ if (atomic_cmpxchg(&key->enabled, 1, 0))
jump_label_update(key);
- jump_label_unlock();
+ else
+ WARN_ON_ONCE(!static_key_slow_try_dec(key));
}
static void __static_key_slow_dec(struct static_key *key)
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