jackd hangs with JackAudioDriver::ProcessGraphAsyncMaster process error after boot

[nettings]

then recovers after > 1min.
May 15 21:42:26 mn-Dantax jackd[643]: JackAudioDriver::ProcessGraphAsyncMaster: Process error
May 15 21:42:26 mn-Dantax jackd[643]: JackAudioDriver::ProcessGraphAsyncMaster: Process error
May 15 21:42:26 mn-Dantax jackd[643]: JackAudioDriver::ProcessGraphAsyncMaster: Process error
[repeats hundreds of times, many per second]

Hardware is Pi4 with DAC+ADC with AMP60 on top, kernel 4.19.97

System software: Raspbian GNU/Linux 10 (buster)
medianet overlay: Tue May 5 23:02:49 2020 +0200 b13c3ad

[nettings]

not 100% reproducible, happened two out of three boots so far.

[nettings]

This was only ever seen on several DAC+ADC, possibly also without AMP60. No further incidents in a while, all machines had a routine kernel update. Needs to be watched, though.
EDIT: apparently unrelated to hardware

[nettings]

same issue on AMP4/RPI4
Linux mn-vp200a-l 5.4.51-v7l+

[nettings]

Usually not self-recovering. Killing mod-host helps most of the time, but not always.

[nettings]

Funny: is this the race condition I'm seeing? There is a jack_lsp job running in parallel...

Sep 10 22:16:10 mn-vp200a-l zita-n2j[646]: Connect: can't connect named futex name = jack_sem.1001_default_lsp-01 err = No such file or directory

This was the first time such a message occurred.

EDIT: since observed two more times, out of some 20 reboots.

[nettings]

It looks like jack client creation and/or jack_lsp is racy and can send jackd into this death spiral. After reintroducing waiting for ports, the issue seems to have disappeared or at least become so rare that I haven't observed it since. Needs to be looked at further. See commits c214689 and 4f55138 .

[nettings]

It still happens when the boot process is stalled because of an fsck or because systemd-timesyncd stalls and then syncs, at which point all Jack clients are long ready and spinning on mn_jack_wait or mn_jack_connect, which seems to trigger the death spiral.

[nettings]

Not observed since.

[nettings]

again observed several times on bookworm, only ever with the mod-monitor client.