ALSA: pcm: Disable bottom softirqs as part of spin_lock_irq() on PREEMPT_RT

snd_pcm_group_lock_irq() acquires a spinlock_t and disables interrupts
via spin_lock_irq(). This also implicitly disables the handling of
softirqs such as TIMER_SOFTIRQ.
On PREEMPT_RT softirqs are preemptible and spin_lock_irq() does not
disable them. That means a timer can be invoked during spin_lock_irq()
on the same CPU. Due to synchronisations reasons local_bh_disable() has
a per-CPU lock named softirq_ctrl.lock which synchronizes individual
softirq against each other.
syz-bot managed to trigger a lockdep report where softirq_ctrl.lock is
acquired in hrtimer_cancel() in addition to hrtimer_run_softirq(). This
is a possible deadlock.

The softirq_ctrl.lock can not be made part of spin_lock_irq() as this
would lead to too much synchronisation against individual threads on the
system. To avoid the possible deadlock, softirqs must be manually
disabled before the lock is acquired.

Disable softirqs before the lock is acquired on PREEMPT_RT.

Reported-by: syzbot+10b4363fb0f46527f3f3@syzkaller.appspotmail.com
Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 1eab940fa2e5ac61b3354f83b12a8210dc6af8ba..68bee40c9adafd9bbb079c058ecf80d14f8885c7 100644 (file)
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -84,19 +84,24 @@ void snd_pcm_group_init(struct snd_pcm_group *group)
 }
 
 /* define group lock helpers */
-#define DEFINE_PCM_GROUP_LOCK(action, mutex_action) \
+#define DEFINE_PCM_GROUP_LOCK(action, bh_lock, bh_unlock, mutex_action) \
 static void snd_pcm_group_ ## action(struct snd_pcm_group *group, bool nonatomic) \
 { \
-       if (nonatomic) \
+       if (nonatomic) { \
                mutex_ ## mutex_action(&group->mutex); \
-       else \
-               spin_ ## action(&group->lock); \
-}
-
-DEFINE_PCM_GROUP_LOCK(lock, lock);
-DEFINE_PCM_GROUP_LOCK(unlock, unlock);
-DEFINE_PCM_GROUP_LOCK(lock_irq, lock);
-DEFINE_PCM_GROUP_LOCK(unlock_irq, unlock);
+       } else { \
+               if (IS_ENABLED(CONFIG_PREEMPT_RT) && bh_lock)   \
+                       local_bh_disable();                     \
+               spin_ ## action(&group->lock);                  \
+               if (IS_ENABLED(CONFIG_PREEMPT_RT) && bh_unlock) \
+                       local_bh_enable();                      \
+       }                                                       \
+}
+
+DEFINE_PCM_GROUP_LOCK(lock, false, false, lock);
+DEFINE_PCM_GROUP_LOCK(unlock, false, false, unlock);
+DEFINE_PCM_GROUP_LOCK(lock_irq, true, false, lock);
+DEFINE_PCM_GROUP_LOCK(unlock_irq, false, true, unlock);
 
 /**
  * snd_pcm_stream_lock - Lock the PCM stream