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SubjectRe: [PATCH] locking/static_key: Fix false positive warnings on concurrent dec/inc
On Tue, Mar 19, 2019 at 10:46:57AM -0700, Jakub Kicinski wrote:

> That indeed looks far cleanest, thanks!
>
> Tested-by: Jakub Kicinski <jakub.kicinski@netronome.com>

Thanks, I've made it into the below patch.

---

Subject: locking/static_key: Fix false positive warnings on concurrent dec/inc
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 19 Mar 2019 13:18:56 +0100

Even though the atomic_dec_and_mutex_lock() in
__static_key_slow_dec_cpuslocked() can never see a negative value in
key->enabled the subsequent sanity check is re-reading key->enabled, which may
have been set to -1 in the meantime by static_key_slow_inc_cpuslocked().


CPU A CPU B

__static_key_slow_dec_cpuslocked(): static_key_slow_inc_cpuslocked():
# enabled = 1
atomic_dec_and_mutex_lock()
# enabled = 0
atomic_read() == 0
atomic_set(-1)
# enabled = -1
val = atomic_read()
# Oops - val == -1!


The test case is TCP's clean_acked_data_enable() / clean_acked_data_disable()
as tickled by ktls (net/ktls).

Cc: mingo@kernel.org
Cc: oss-drivers@netronome.com
Cc: ard.biesheuvel@linaro.org
Cc: tglx@linutronix.de
Cc: pbonzini@redhat.com
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/jump_label.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -206,6 +206,8 @@ static void __static_key_slow_dec_cpuslo
unsigned long rate_limit,
struct delayed_work *work)
{
+ int val;
+
lockdep_assert_cpus_held();

/*
@@ -215,17 +217,20 @@ static void __static_key_slow_dec_cpuslo
* returns is unbalanced, because all other static_key_slow_inc()
* instances block while the update is in progress.
*/
- if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
- WARN(atomic_read(&key->enabled) < 0,
- "jump label: negative count!\n");
+ val = atomic_fetch_add_unless(&key->enabled, -1, 1);
+ if (val != 1) {
+ WARN(val < 0, "jump label: negative count!\n");
return;
}

- if (rate_limit) {
- atomic_inc(&key->enabled);
- schedule_delayed_work(work, rate_limit);
- } else {
- jump_label_update(key);
+ jump_label_lock();
+ if (atomic_dec_and_test(&key->enabled)) {
+ if (rate_limit) {
+ atomic_inc(&key->enabled);
+ schedule_delayed_work(work, rate_limit);
+ } else {
+ jump_label_update(key);
+ }
}
jump_label_unlock();
}
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