Hi,
On Fri, Jul 02, 2021 at 10:19:46PM -0700, Linus Torvalds wrote:
On Fri, Jul 2, 2021 at 9:37 PM Linus Torvalds torvalds@linux-foundation.org wrote:
But I thought I'd report this as a likely candidate.
Confirmed. The watchdog hang bisects right down to commit 9ce650a75a3b ("ALSA: usb-audio: Reduce latency at playback start").
And reverting it on top of my tree also fixes the hang, so it's not some bisection fluke.
I have no idea what is actually wrong with that commit, but it most definitely is the problem, and I have reverted it in my tree so that I can continue merging stuff tomorrow.
The cause seems to be the attempt to lock PCM substream recursively introduced by the issued commit.
Would I ask you to test with below patch? I apologize that the patch is still untested in my side since at present I have no preparation to debug USB stuffs instantly (I'm just a maintainer for ALSA firewire stack...), so I'm glad if getting your cooperation for the issue.
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From f7ab449f10152635ad7083aa73d80e3fb1adabb4 Mon Sep 17 00:00:00 2001
From: Takashi Sakamoto o-takashi@sakamocchi.jp Date: Sat, 3 Jul 2021 15:23:25 +0900 Subject: [PATCH] ALSA: usb-audio: fix recursive lock of PCM substream when starting playback PCM substream
A commit 9ce650a75a3b ("ALSA: usb-audio: Reduce latency at playback start") unfortunately introduced the call of snd_pcm_period_elapsed() under acquired lock of PCM substream. This causes recursive lock and results in dead-lock.
->ioctl(2) (sound/core/pcm_native.c) ->snd_pcm_stream_lock_irqsave() <- ... ->struct snd_pcm_ops.trigger() (sound/usb/pcm.c) = snd_usb_substream_playback_trigger() ->start_endpoints() (sound/usb/endpoint.c) ->snd_usb_endpoint_start() ->prepare_outbound_urb() ->struct snd_usb_endpoint.prepare_data_urb() (sound/usb/pcm.c) = prepare_playback_urb() (sound/core/pcm_lib.c) ->snd_pcm_period_elapsed() (sound/core/pcm_native.c) ->snd_pcm_stream_lock_irqsave() <-
This commit fixes the issue to use newly added function; snd_pcm_period_elapsed_under_stream_lock() with condition to check running context.
Reported-by: Hector Martin marcan@marcan.st Fixes: 9ce650a75a3b ("ALSA: usb-audio: Reduce latency at playback start") Signed-off-by: Takashi Sakamoto o-takashi@sakamocchi.jp --- sound/usb/pcm.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index c66831ee15f9..235070f0236a 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -1395,8 +1395,16 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
spin_unlock_irqrestore(&subs->lock, flags); urb->transfer_buffer_length = bytes; - if (period_elapsed) - snd_pcm_period_elapsed(subs->pcm_substream); + if (period_elapsed) { + // The callback of struct snd_pcm_ops.trigger with SNDRV_PCM_TRIGGER_START command + // can reach here, under acquired lock of PCM substream. To avoid dead-lock, check + // current context and call corresponding function. + if (in_softirq()) { + snd_pcm_period_elapsed(subs->pcm_substream); + } else { + snd_pcm_period_elapsed_under_stream_lock(subs->pcm_substream); + } + } }
/*