linux-kernel - [PATCH 4.19 016/247] locking/static_key: Fix false positive warnings on concurrent dec/inc

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Message-Id: <20210301161032.484926340@linuxfoundation.org>
Date:   Mon,  1 Mar 2021 17:10:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>, ard.biesheuvel@...aro.org,
        oss-drivers@...ronome.com, pbonzini@...hat.com,
        Ingo Molnar <mingo@...nel.org>,
        Will McVicker <willmcvicker@...gle.com>
Subject: [PATCH 4.19 016/247] locking/static_key: Fix false positive warnings on concurrent dec/inc

From: Peter Zijlstra <peterz@...radead.org>

commit a1247d06d01045d7ab2882a9c074fbf21137c690 upstream.

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).

Suggested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Reported-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Tested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will.deacon@....com>
Cc: ard.biesheuvel@...aro.org
Cc: oss-drivers@...ronome.com
Cc: pbonzini@...hat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Will McVicker <willmcvicker@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/jump_label.c |   21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -186,6 +186,8 @@ static void __static_key_slow_dec_cpuslo
 					   unsigned long rate_limit,
 					   struct delayed_work *work)
 {
+	int val;
+
 	lockdep_assert_cpus_held();
 
 	/*
@@ -195,17 +197,20 @@ 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();
 }