[PATCH] ALSA: firewire-lib: restore process context workqueue to prevent deadlock

[Edmund Raile <edmund.raile@xxxxxxxxx>]

Commit b5b519965c4c ("ALSA: firewire-lib: operate for period elapse event
in process context") removed the process context workqueue from
amdtp_domain_stream_pcm_pointer() and update_pcm_pointers() to remove
its overhead.

With RME Fireface 800, this lead to a regression since Kernels 5.14.0,
causing a deadlock with eventual system freeze under ALSA operation:

? tasklet_unlock_spin_wait
 </NMI>
 <TASK>
ohci_flush_iso_completions firewire_ohci
amdtp_domain_stream_pcm_pointer snd_firewire_lib
snd_pcm_update_hw_ptr0 snd_pcm
snd_pcm_status64 snd_pcm

? native_queued_spin_lock_slowpath
 </NMI>
 <IRQ>
_raw_spin_lock_irqsave
snd_pcm_period_elapsed snd_pcm
process_rx_packets snd_firewire_lib
irq_target_callback snd_firewire_lib
handle_it_packet firewire_ohci
context_tasklet firewire_ohci

Restore the work queue to prevent deadlock between ALSA substream
process context spin_lock of snd_pcm_stream_lock_irq() in snd_pcm_status64()
and OHCI 1394 IT softIRQ context spin_lock of snd_pcm_stream_lock_irqsave()
in snd_pcm_period_elapsed().

to reproduce the issue:
direct ALSA playback to the device:
  mpv --audio-device=alsa/sysdefault:CARD=Fireface800 Spor-Ignition.flac
Time to occurrence: 2s to 30m
Likelihood increased by:
  - high CPU load
    stress --cpu $(nproc)
  - switching between applications via workspaces
    tested with i915 in Xfce
PulsaAudio / PipeWire conceal the issue as they run PCM substream
without period wakeup mode, issuing less hardIRQs.

Closes: https://lore.kernel.org/regressions/kwryofzdmjvzkuw6j3clftsxmoolynljztxqwg76hzeo4simnl@jn3eo7pe642q/T/#u
Fixes: 7ba5ca32fe6e ("ALSA: firewire-lib: operate for period elapse event in process context")
Signed-off-by: Edmund Raile <edmund.raile@xxxxxxxxx>
---
This is the follow-up patch to the 5.14.0 regression I reported:
https://lore.kernel.org/regressions/kwryofzdmjvzkuw6j3clftsxmoolynljztxqwg76hzeo4simnl@jn3eo7pe642q/T/#u
("[REGRESSION] ALSA: firewire-lib: snd_pcm_period_elapsed deadlock with Fireface 800")

Takashi Sakamoto explained the issue in his response to the regression:
A. In the process context
    * (lock A) Acquiring spin_lock by snd_pcm_stream_lock_irq() in
               snd_pcm_status64()
    * (lock B) Then attempt to enter tasklet

B. In the softIRQ context
    * (lock B) Enter tasklet
    * (lock A) Attempt to acquire spin_lock by snd_pcm_stream_lock_irqsave() in
               snd_pcm_period_elapsed()

This leads me to believe this isn't just an issue limited to the RME Fireface

driver (snd_fireface), though I can not test the other devices.
 sound/firewire/amdtp-stream.c | 32 +++++++++++++++++++++-----------
 sound/firewire/amdtp-stream.h |  1 +
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index d35d0a420ee0..77b99a2117f4 100644
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -77,6 +77,8 @@
 // overrun. Actual device can skip more, then this module stops the packet streaming.
 #define IR_JUMBO_PAYLOAD_MAX_SKIP_CYCLES	5
 
+static void pcm_period_work(struct work_struct *work);
+
 /**
  * amdtp_stream_init - initialize an AMDTP stream structure
  * @s: the AMDTP stream to initialize
@@ -105,6 +107,7 @@ int amdtp_stream_init(struct amdtp_stream *s, struct fw_unit *unit,
 	s->flags = flags;
 	s->context = ERR_PTR(-1);
 	mutex_init(&s->mutex);
+	INIT_WORK(&s->period_work, pcm_period_work);
 	s->packet_index = 0;
 
 	init_waitqueue_head(&s->ready_wait);
@@ -347,6 +350,7 @@ EXPORT_SYMBOL(amdtp_stream_get_max_payload);
  */
 void amdtp_stream_pcm_prepare(struct amdtp_stream *s)
 {
+	cancel_work_sync(&s->period_work);
 	s->pcm_buffer_pointer = 0;
 	s->pcm_period_pointer = 0;
 }
@@ -611,19 +615,21 @@ static void update_pcm_pointers(struct amdtp_stream *s,
 		// The program in user process should periodically check the status of intermediate
 		// buffer associated to PCM substream to process PCM frames in the buffer, instead
 		// of receiving notification of period elapsed by poll wait.
-		if (!pcm->runtime->no_period_wakeup) {
-			if (in_softirq()) {
-				// In software IRQ context for 1394 OHCI.
-				snd_pcm_period_elapsed(pcm);
-			} else {
-				// In process context of ALSA PCM application under acquired lock of
-				// PCM substream.
-				snd_pcm_period_elapsed_under_stream_lock(pcm);
-			}
-		}
+		if (!pcm->runtime->no_period_wakeup)
+			queue_work(system_highpri_wq, &s->period_work);
 	}
 }
 
+static void pcm_period_work(struct work_struct *work)
+{
+	struct amdtp_stream *s = container_of(work, struct amdtp_stream,
+					      period_work);
+	struct snd_pcm_substream *pcm = READ_ONCE(s->pcm);
+	
+	if (pcm)
+		snd_pcm_period_elapsed(pcm);
+}
+
 static int queue_packet(struct amdtp_stream *s, struct fw_iso_packet *params,
 			bool sched_irq)
 {
@@ -1854,7 +1860,10 @@ unsigned long amdtp_domain_stream_pcm_pointer(struct amdtp_domain *d,
 	if (irq_target && amdtp_stream_running(irq_target)) {
 		// In software IRQ context, the call causes dead-lock to disable the tasklet
 		// synchronously.
-		if (!in_softirq())
+		// workqueue serves to prevent deadlock between process context spinlock
+		// of snd_pcm_stream_lock_irq() in snd_pcm_status64() and softIRQ spinlock
+		// of snd_pcm_stream_lock_irqsave() in snd_pcm_period_elapsed()
+		if ((!in_softirq()) && (current_work() != &s->period_work))
 			fw_iso_context_flush_completions(irq_target->context);
 	}
 
@@ -1910,6 +1919,7 @@ static void amdtp_stream_stop(struct amdtp_stream *s)
 		return;
 	}
 
+	cancel_work_sync(&s->period_work);
 	fw_iso_context_stop(s->context);
 	fw_iso_context_destroy(s->context);
 	s->context = ERR_PTR(-1);
diff --git a/sound/firewire/amdtp-stream.h b/sound/firewire/amdtp-stream.h
index a1ed2e80f91a..775db3fc4959 100644
--- a/sound/firewire/amdtp-stream.h
+++ b/sound/firewire/amdtp-stream.h
@@ -191,6 +191,7 @@ struct amdtp_stream {
 
 	/* For a PCM substream processing. */
 	struct snd_pcm_substream *pcm;
+	struct work_struct period_work;
 	snd_pcm_uframes_t pcm_buffer_pointer;
 	unsigned int pcm_period_pointer;
 	unsigned int pcm_frame_multiplier;
-- 
2.45.2