Re: [PATCH] locking/static_key: Fix false positive warnings on concurrent dec/inc

[Peter Zijlstra]

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@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
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@xxxxxxxxxx
Cc: oss-drivers@xxxxxxxxxxxxx
Cc: ard.biesheuvel@xxxxxxxxxx
Cc: tglx@xxxxxxxxxxxxx
Cc: pbonzini@xxxxxxxxxx
Reported-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
Suggested-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
Tested-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 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();
 }