[PATCH v2 2/2] ALSA: firewire-lib: restore process context workqueue to prevent deadlock

[Edmund Raile <edmund.raile@xxxxxxxxxxxxxx>]

Commit 7ba5ca32fe6e ("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:

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()

? 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 process context 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().

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 7ba5ca32fe6e ("ALSA: firewire-lib: operate for period elapse event in process context")
Link: https://lore.kernel.org/r/kwryofzdmjvzkuw6j3clftsxmoolynljztxqwg76hzeo4simnl@jn3eo7pe642q/
Reported-by: edmund.raile <edmund.raile@xxxxxxxxx>
Signed-off-by: Edmund Raile <edmund.raile@xxxxxxxxxxxxxx>
---
 sound/firewire/amdtp-stream.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index 31201d506a21..c037d7de1fdc 100644
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -615,16 +615,9 @@ 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)
+			// wq used with amdtp_domain_stream_pcm_pointer() to prevent deadlock
+			queue_work(system_highpri_wq, &s->period_work);
 	}
 }
 
@@ -1866,9 +1859,11 @@ unsigned long amdtp_domain_stream_pcm_pointer(struct amdtp_domain *d,
 
 	// Process isochronous packets queued till recent isochronous cycle to handle PCM frames.
 	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())
+		// use wq to prevent deadlock between process context spin_lock
+		// of snd_pcm_stream_lock_irq() in snd_pcm_status64() and
+		// softIRQ context spin_lock 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);
 	}
 
-- 
2.45.2