Patch "ALSA: hrtimer: Fix stall by hrtimer_cancel()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ALSA: hrtimer: Fix stall by hrtimer_cancel()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
alsa-hrtimer-fix-stall-by-hrtimer_cancel.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Takashi Iwai <[email protected]>
Date: Mon, 18 Jan 2016 13:52:47 +0100
Subject: ALSA: hrtimer: Fix stall by hrtimer_cancel()

From: Takashi Iwai <[email protected]>

commit 2ba1fe7a06d3624f9a7586d672b55f08f7c670f3 upstream.

hrtimer_cancel() waits for the completion from the callback, thus it must not be called inside the callback itself. This was already a problem in the past with ALSA hrtimer driver, and the early commit [fcfdebe70759: ALSA: hrtimer - Fix lock-up] tried to address it.

However, the previous fix is still insufficient: it may still cause a lockup when the ALSA timer instance reprograms itself in its callback. Then it invokes the start function even in snd_timer_interrupt() that is called in hrtimer callback itself, results in a CPU stall. This is no hypothetical problem but actually triggered by syzkaller fuzzer.

This patch tries to fix the issue again. Now we call hrtimer_try_to_cancel() at both start and stop functions so that it won't fall into a deadlock, yet giving some chance to cancel the queue if the functions have been called outside the callback. The proper hrtimer_cancel() is called in anyway at closing, so this should be enough.

---
 sound/core/hrtimer.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -90,7 +90,7 @@ static int snd_hrtimer_start(struct snd_
 	struct snd_hrtimer *stime = t->private_data;
 
 	atomic_set(&stime->running, 0);
-	hrtimer_cancel(&stime->hrt);
+	hrtimer_try_to_cancel(&stime->hrt);
 	hrtimer_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
 		      HRTIMER_MODE_REL);
 	atomic_set(&stime->running, 1);

@@ -101,6 +101,7 @@ static int snd_hrtimer_stop(struct snd_t
 {
 	struct snd_hrtimer *stime = t->private_data;
 	atomic_set(&stime->running, 0);
+	hrtimer_try_to_cancel(&stime->hrt);
 	return 0;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/alsa-hrtimer-fix-stall-by-hrtimer_cancel.patch queue-4.4/alsa-hda-fix-white-noise-on-dell-latitude-e5550.patch queue-4.4/alsa-hda-fix-the-headset-mic-detection-problem-for-a-dell-laptop.patch queue-4.4/alsa-timer-harden-slave-timer-list-handling.patch queue-4.4/alsa-timer-fix-double-unlink-of-active_list.patch queue-4.4/alsa-control-avoid-kernel-warnings-from-tlv-ioctl-with-numid-0.patch queue-4.4/alsa-hda-flush-the-pending-probe-work-at-remove.patch queue-4.4/alsa-hda-fixup-inverted-internal-mic-for-lenovo-e50-80.patch queue-4.4/alsa-usb-add-native-dsd-support-for-oppo-ha-1.patch queue-4.4/alsa-hda-fix-missing-module-loading-with-model-generic-option.patch queue-4.4/alsa-seq-fix-race-at-timer-setup-and-close.patch queue-4.4/alsa-pcm-fix-snd_pcm_hw_params-struct-copy-in-compat-mode.patch queue-4.4/alsa-seq-fix-snd_seq_call_port_info_ioctl-in-compat-mode.patch queue-4.4/alsa-seq-fix-missing-null-check-at-remove_events-ioctl.patch queue-4.4/alsa-usb-audio-fix-mixer-ctl-regression-of-native-instrument-devices.patch queue-4.4/alsa-timer-handle-disconnection-more-safely.patch queue-4.4/alsa-hda-fix-bass-pin-fixup-for-asus-n550jx.patch queue-4.4/alsa-timer-fix-race-among-timer-ioctls.patch queue-4.4/alsa-hda-add-fixup-for-dell-latitidue-e6540.patch queue-4.4/alsa-usb-audio-avoid-calling-usb_autopm_put_interface-at-disconnect.patch