From: Thomas Gleixner Date: Fri, 1 Mar 2013 11:17:42 +0100 Subject: futex: Ensure lock/unlock symetry versus pi_lock and hash bucket lock Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.1-rt3.tar.xz In exit_pi_state_list() we have the following locking construct: spin_lock(&hb->lock); raw_spin_lock_irq(&curr->pi_lock); ... spin_unlock(&hb->lock); In !RT this works, but on RT the migrate_enable() function which is called from spin_unlock() sees atomic context due to the held pi_lock and just decrements the migrate_disable_atomic counter of the task. Now the next call to migrate_disable() sees the counter being negative and issues a warning. That check should be in migrate_enable() already. Fix this by dropping pi_lock before unlocking hb->lock and reaquire pi_lock after that again. This is safe as the loop code reevaluates head again under the pi_lock. Reported-by: Yong Zhang Signed-off-by: Thomas Gleixner Signed-off-by: Sebastian Andrzej Siewior --- kernel/futex.c | 2 ++ 1 file changed, 2 insertions(+) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -918,7 +918,9 @@ void exit_pi_state_list(struct task_stru if (head->next != next) { /* retain curr->pi_lock for the loop invariant */ raw_spin_unlock(&pi_state->pi_mutex.wait_lock); + raw_spin_unlock_irq(&curr->pi_lock); spin_unlock(&hb->lock); + raw_spin_lock_irq(&curr->pi_lock); put_pi_state(pi_state); continue; }