On Tue, 12 Nov 2019 07:51:06 +0100, Henry Lin wrote:
While output urb's snd_complete_urb() is executing, calling prepare_outbound_urb() may cause endpoint stopped before prepare_outbound_urb() returns and result in next urb submitted to stopped endpoint. usb-audio driver cannot re-use it afterwards as the urb is still hold by usb stack.
This change checks EP_FLAG_RUNNING flag after prepare_outbound_urb() again to let snd_complete_urb() know the endpoint already stopped and does not submit next urb.
OK, this part looks good and understandable.
We observed two scenario have this issue:
- While executing snd_complete_urb() to complete an output urb, calling prepare_outbound_urb() let deactive_urbs() get called to unlink all active urbs.
[ 268.097066] [<ffffffc000af7638>] deactivate_urbs+0xd4/0x108 [ 268.102633] [<ffffffc000af87fc>] snd_usb_endpoint_stop+0x30/0x58 [ 268.108636] [<ffffffc000b0272c>] snd_usb_substream_playback_trigger+0xa4/0xf4 [ 268.115765] [<ffffffc000acdbd0>] snd_pcm_do_stop+0x4c/0x58 [ 268.121245] [<ffffffc000acda24>] snd_pcm_action_single+0x40/0x88 [ 268.127245] [<ffffffc000ace984>] snd_pcm_action+0x30/0xf0 [ 268.132632] [<ffffffc000acea68>] snd_pcm_stop+0x24/0x2c [ 268.137851] [<ffffffc000ad5e14>] xrun+0x60/0x6c [ 268.142374] [<ffffffc000ad7a98>] snd_pcm_update_state+0xa8/0x10c [ 268.148374] [<ffffffc000ad7e24>] snd_pcm_update_hw_ptr0+0x328/0x344 [ 268.154635] [<ffffffc000ad7ed8>] snd_pcm_period_elapsed+0x98/0xb0 [ 268.160723] [<ffffffc000b02510>] prepare_playback_urb+0x46c/0x488 [ 268.166810] [<ffffffc000af7d60>] prepare_outbound_urb+0x60/0x1d4 [ 268.172805] [<ffffffc000af8d60>] snd_complete_urb+0x244/0x264 [ 268.178548] [<ffffffc00081fb38>] __usb_hcd_giveback_urb+0x94/0x104 [ 268.184721] [<ffffffc00081fbe4>] usb_hcd_giveback_urb+0x3c/0x114 [ 268.190724] [<ffffffc00084d4b4>] handle_tx_event+0x1304/0x1434 [ 268.196552] [<ffffffc00084dbc0>] xhci_handle_event+0x5dc/0x788 [ 268.202378] [<ffffffc00084dee4>] xhci_irq+0x178/0x280
- Userspace application stops playback from sound subsystem with below call stack:
[ 28.506477] CPU: 5 PID: 1274 Comm: AudioOut_25 Not tainted 4.4.38-tegra #31 [ 28.513430] Hardware name: quill (DT) [ 28.517085] Call trace: [ 28.519531] [<ffffffc000089a84>] dump_backtrace+0x0/0xf8 [ 28.524837] [<ffffffc000089c44>] show_stack+0x14/0x1c [ 28.529885] [<ffffffc000401c54>] dump_stack+0xac/0xe0 [ 28.534931] [<ffffffc000b35f94>] deactivate_urbs+0x148/0x180 [ 28.540578] [<ffffffc000b37160>] snd_usb_endpoint_stop+0x30/0x58 [ 28.546571] [<ffffffc000b410d8>] snd_usb_substream_playback_trigger+0xa4/0xf4 [ 28.553699] [<ffffffc000b0c160>] snd_pcm_do_stop+0x4c/0x58 [ 28.559179] [<ffffffc000b0bfb4>] snd_pcm_action_single+0x40/0x88 [ 28.565178] [<ffffffc000b0cf14>] snd_pcm_action+0x30/0xf0 [ 28.570568] [<ffffffc000b0fbc8>] snd_pcm_drop+0xac/0x140 [ 28.575873] [<ffffffc000b0fc84>] snd_pcm_release_substream+0x28/0xb0 [ 28.582212] [<ffffffc000b0fd48>] snd_pcm_release+0x3c/0x98 [ 28.587686] [<ffffffc0001e3210>] __fput+0xe0/0x1ac [ 28.592469] [<ffffffc0001e3334>] ____fput+0xc/0x14 [ 28.597253] [<ffffffc0000c2904>] task_work_run+0xa0/0xc0 [ 28.602558] [<ffffffc0000897bc>] do_notify_resume+0x48/0x60 [ 28.608123] [<ffffffc000084ee8>] work_pending+0x1c/0x20
In the call path, snd_pcm_stream spinlock has been acquired in snd_pcm_drop(). If an output urb is completed between the spinlock acquired and deactivate_urbs() clears EP_FLAG_RUNNING for the endpoint, its executing of snd_complete_urb() will be blocked for acquiring snd_pcm_stream spinlock in snd_pcm_period_elapsed() until the lock is released in snd_pcm_drop(). When snd_complete_urb() continues, all jobs for deactivate_urbs() are finished.
... but this part is unclear to me. Do you mean that we have a deadlock in these two concurrent calls without your patch?
Thanks!
Takashi