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fix(alsa): skip drain() on non-Running PCM to prevent USB kernel deadlock #1691

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fix(alsa): skip drain() on non-Running PCM to prevent USB kernel deadlock #1691

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31f3647
fix(alsa): skip drain() on non-Running PCM to prevent USB kernel dead…
zeemawn Mar 7, 2026
b112f80
style: fix import ordering for rustfmt
zeemawn Mar 7, 2026
816a488
fix: add debug log when drain is skipped due to non-Running PCM state
zeemawn Mar 7, 2026
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11 changes: 9 additions & 2 deletions playback/src/audio_backend/alsa.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@ use crate::convert::Converter;
use crate::decoder::AudioPacket;
use crate::{NUM_CHANNELS, SAMPLE_RATE};
use alsa::device_name::HintIter;
use alsa::pcm::{Access, Format, Frames, HwParams, PCM};
use alsa::pcm::{Access, Format, Frames, HwParams, State, PCM};
use alsa::{Direction, ValueOr};
use std::process::exit;
use thiserror::Error;
Expand Down Expand Up @@ -442,7 +442,14 @@ impl Sink for AlsaSink {

let pcm = self.pcm.take().ok_or(AlsaError::NotConnected)?;

pcm.drain().map_err(AlsaError::DrainFailure)?;
// Only drain if the PCM is in Running state. After a write error,
// try_recover() leaves the PCM in Prepared state, and calling drain()
// on a Prepared-state USB PCM can deadlock the kernel driver
// (confirmed on Raspberry Pi 3 with the dwc_otg USB controller).
// In that case, dropping the PCM is sufficient to release resources.
if pcm.state() == State::Running {
pcm.drain().map_err(AlsaError::DrainFailure)?;
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Copilot AI Mar 7, 2026

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When drain() is skipped because the PCM is not in Running state, there is no log message emitted. In the error path described in the PR (after a broken pipe triggers try_recover()), this means the skip happens silently. Since the codebase already uses warn! to log the original write error on line 503, it would be helpful to also add a debug! or warn! here to indicate that drain was intentionally skipped and the PCM will be closed by dropping — for example: "PCM not in Running state (state: {:?}), skipping drain and dropping PCM", pcm.state(). This would make diagnosing future issues on other hardware much easier.

Suggested change
if pcm.state() == State::Running {
pcm.drain().map_err(AlsaError::DrainFailure)?;
let state = pcm.state();
if state == State::Running {
pcm.drain().map_err(AlsaError::DrainFailure)?;
} else {
debug!(
"PCM not in Running state (state: {:?}), skipping drain and dropping PCM",
state
);

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@zeemawn zeemawn Mar 7, 2026

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Good call — added in 816a488.

}
}

Ok(())
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