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

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

[edmund.raile]

> Thank you for your sending the revised patches, it looks better than the
> previous one. However, I have an additional request.
Allright, patch v3 it is.

> [1] https://git-scm.com/docs/git-revert
Should have known git has something like that, how handy!

> $ git revert -s b5b519965c4c
Yes, 5b5 can be removed via revert, but what is the difference in
effect? Just time saving?
> $ git revert -s 7ba5ca32fe6e
This one I'd like to ask you about:
The original inline comment in amdtp-stream.c
amdtp_domain_stream_pcm_pointer()
```
// This function is called in software IRQ context of
// period_work or process context.
//
// When the software IRQ context was scheduled by software IRQ
// context of IT contexts, queued packets were already handled.
// Therefore, no need to flush the queue in buffer furthermore.
//
// When the process context reach here, some packets will be
// already queued in the buffer. These packets should be handled
// immediately to keep better granularity of PCM pointer.
//
// Later, the process context will sometimes schedules software
// IRQ context of the period_work. Then, no need to flush the
// queue by the same reason as described in the above
```
(let's call the above v1) was replaced with
```
// In software IRQ context, the call causes dead-lock to disable the tasklet
// synchronously.
```
on occasion of 7ba5ca32fe6e (let's call this v2).

I sought to replace it with
```
// 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()
```
to prevent this issue from occurring again (let's call this v3).

Should I include v1, v3 or a combination of v1 and v3 in my next patch?

> Just for safe, it is preferable to execute 'scripts/checkpatch.pl' in
> kernel tree to check the patchset generated by send-email subcommand[3].
Absolutely should have done so, sorry.

Thank you for your patience and guidance,
Edmund Raile.

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

[Takashi Sakamoto]

On Mon, Jul 29, 2024 at 10:16:04AM +0000, edmund.raile wrote:
> > $ git revert -s b5b519965c4c
> Yes, 5b5 can be removed via revert, but what is the difference in
> effect? Just time saving?

The title of commit generated by git-revert could be helpful when reading
history of changes later. The code change is itself important, while the
history of changes often assists developers to work between several
trees.

> > $ git revert -s 7ba5ca32fe6e
> This one I'd like to ask you about:
> The original inline comment in amdtp-stream.c
> amdtp_domain_stream_pcm_pointer()
> ```
> // This function is called in software IRQ context of
> // period_work or process context.
> //
> // When the software IRQ context was scheduled by software IRQ
> // context of IT contexts, queued packets were already handled.
> // Therefore, no need to flush the queue in buffer furthermore.
> //
> // When the process context reach here, some packets will be
> // already queued in the buffer. These packets should be handled
> // immediately to keep better granularity of PCM pointer.
> //
> // Later, the process context will sometimes schedules software
> // IRQ context of the period_work. Then, no need to flush the
> // queue by the same reason as described in the above
> ```
> (let's call the above v1) was replaced with
> ```
> // In software IRQ context, the call causes dead-lock to disable the tasklet
> // synchronously.
> ```
> on occasion of 7ba5ca32fe6e (let's call this v2).
> 
> I sought to replace it with
> ```
> // 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()
> ```
> to prevent this issue from occurring again (let's call this v3).
> 
> Should I include v1, v3 or a combination of v1 and v3 in my next patch?

I think we pay enough attention to regenerate the deadlock, thus v3 is a
good choice. But the cause of deadlock in the above description is a bit
wrong. It is typical AB/BA deadlock, like:

Thread 0:
1. Acquire stream lock by calling 'snd_pcm_stream_lock_irq()' or so
2. Wait until running tasklet finishes by calling 'tasklet_disable_in_atomic()'
   (actually at tasklet_unlock_spin_wait())

Thread 1:
1. Launch tasklet
2. Wait until the stream lock released

The softIRQ context does not related to any type of lock in sound
subsystem essentially.


Thanks

Takashi Sakamoto

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

[Edmund Raile]

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

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

[Edmund Raile]

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

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

[Takashi Sakamoto]

Hi,

On Sun, Jul 28, 2024 at 12:26:21PM +0000, Edmund Raile wrote:
> This patchset serves to prevent a deadlock between
> process context and softIRQ context:

(snip)

> 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(-)

Thank you for your sending the revised patches, it looks better than the
previous one. However, I have an additional request.

In this case, it is enough to execute 'revert' subcommand[1] of git(1),
like:

$ git revert -s b5b519965c4c
$ git revert -s 7ba5ca32fe6e

It is permitted to add postscript to the commit comment generated by the
above command. You see my recent post as an example[2].

Just for safe, it is preferable to execute 'scripts/checkpatch.pl' in
kernel tree to check the patchset generated by send-email subcommand[3].

[1] https://git-scm.com/docs/git-revert
[2] https://lore.kernel.org/lkml/20240725161648.130404-1-o-takashi@sakamocchi.jp/
[3] https://git-scm.com/docs/git-send-email


Thanks

Takashi Sakamoto