[PATCH v4 2/2] Revert "ALSA: firewire-lib: operate for period elapse event in process context"

[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 an AB/BA deadlock competition for the
substream lock with eventual system freeze under ALSA operation:

thread 0:
    * (lock A) acquire substream lock by
	snd_pcm_stream_lock_irq() in
	snd_pcm_status64()
    * (lock B) wait for tasklet to finish by calling
    	tasklet_unlock_spin_wait() in
	tasklet_disable_in_atomic() in
	ohci_flush_iso_completions() of ohci.c

thread 1:
    * (lock B) enter tasklet
    * (lock A) attempt to acquire substream lock,
    	waiting for it to be released:
	snd_pcm_stream_lock_irqsave() in
    	snd_pcm_period_elapsed() in
	update_pcm_pointers() in
	process_ctx_payloads() in
	process_rx_packets() of amdtp-stream.c

? 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
AB/BA deadlock competition for ALSA substream lock of
snd_pcm_stream_lock_irq() in snd_pcm_status64()
and snd_pcm_stream_lock_irqsave() in snd_pcm_period_elapsed().

revert commit 7ba5ca32fe6e ("ALSA: firewire-lib: operate for period
elapse event in process context")

Replace inline description to prevent future deadlock.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 7ba5ca32fe6e ("ALSA: firewire-lib: operate for period elapse event in process context")
Reported-by: edmund.raile <edmund.raile@xxxxxxxxx>
Closes: https://lore.kernel.org/r/kwryofzdmjvzkuw6j3clftsxmoolynljztxqwg76hzeo4simnl@jn3eo7pe642q/
Signed-off-by: Edmund Raile <edmund.raile@xxxxxxxxxxxxxx>
---
 sound/firewire/amdtp-stream.c | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index 31201d506a21..7438999e0510 100644
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -615,16 +615,8 @@ 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);
 	}
 }
 
@@ -1864,11 +1856,14 @@ unsigned long amdtp_domain_stream_pcm_pointer(struct amdtp_domain *d,
 {
 	struct amdtp_stream *irq_target = d->irq_target;
 
-	// 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 AB/BA deadlock competition for
+		// substream lock:
+		// fw_iso_context_flush_completions() acquires
+		// lock by ohci_flush_iso_completions(),
+		// amdtp-stream process_rx_packets() attempts to
+		// acquire same lock by snd_pcm_elapsed()
+		if (current_work() != &s->period_work)
 			fw_iso_context_flush_completions(irq_target->context);
 	}
 
-- 
2.45.2