[PATCH v2 0/2] ALSA: firewire-lib: restore process context workqueue to prevent deadlock

[Edmund Raile <edmund.raile@xxxxxxxxxxxxxx>]

This patchset serves to prevent a deadlock between
process context and softIRQ context:

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

The issue has been reported as a regression of kernel 5.14:
Link: https://lore.kernel.org/regressions/kwryofzdmjvzkuw6j3clftsxmoolynljztxqwg76hzeo4simnl@jn3eo7pe642q/T/#u
("[REGRESSION] ALSA: firewire-lib: snd_pcm_period_elapsed deadlock with
Fireface 800")

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.
Commit b5b519965c4c ("ALSA: firewire-lib: obsolete workqueue for period
update") belongs to the same patch series and removed
the now-unused workqueue entirely.

Though being observed on RME Fireface 800, this issue would affect all
Firewire audio interfaces using ohci amdtp + pcm streaming.

ALSA streaming, especially under intensive CPU load will reveal this issue
the soonest due to issuing more hardIRQs, with time to occurrence ranging
from 2 secons to 30 minutes after starting playback.

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.

Backport note:
Also applies to and fixes on (tested):
6.10.2, 6.9.12, 6.6.43, 6.1.102, 5.15.164

Edmund Raile (2):
  ALSA: firewire-lib: restore workqueue for process context
  ALSA: firewire-lib: prevent deadlock between process and softIRQ
    context

 sound/firewire/amdtp-stream.c | 36 ++++++++++++++++++++++-------------
 sound/firewire/amdtp-stream.h |  1 +
 2 files changed, 24 insertions(+), 13 deletions(-)

-- 
2.45.2