Commit a1247d06 authored Mar 19, 2019 by Peter Zijlstra Committed by Ingo Molnar Apr 03, 2019

locking/static_key: Fix false positive warnings on concurrent dec/inc



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@netronome.com>
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: ard.biesheuvel@linaro.org
Cc: oss-drivers@netronome.com
Cc: pbonzini@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 5e7a8ca3

kernel/jump_label.c

+13 −8

Original line number	Original line	Diff line number	Diff line
	@@ -206,6 +206,8 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key,
	unsigned long rate_limit,		unsigned long rate_limit,
	struct delayed_work *work)		struct delayed_work *work)
	{		{
			int val;

	lockdep_assert_cpus_held();		lockdep_assert_cpus_held();

	/*		/*
	@@ -215,18 +217,21 @@ static void __static_key_slow_dec_cpuslocked(struct static_key *key,
	* returns is unbalanced, because all other static_key_slow_inc()		* returns is unbalanced, because all other static_key_slow_inc()
	* instances block while the update is in progress.		* instances block while the update is in progress.
	*/		*/
	if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {		val = atomic_fetch_add_unless(&key->enabled, -1, 1);
	WARN(atomic_read(&key->enabled) < 0,		if (val != 1) {
	"jump label: negative count!\n");		WARN(val < 0, "jump label: negative count!\n");
	return;		return;
	}		}

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