The patch titled sound/core/pcm_lib.c: fix race condition in wait_for_avail() has been added to the -mm tree. Its filename is sound-core-pcm_libc-fix-race-condition-in-wait_for_avail.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: sound/core/pcm_lib.c: fix race condition in wait_for_avail() From: Arjan van de Ven <arjan@xxxxxxxxxxxxx> wait_for_avail() in pcm_lib.c has a race in it (observed in practice by an Intel validation group). The function is supposed to return once space in the buffer has become available, or if some timeout happens. The entity that creates space (irq handler of sound driver and some such) will do a wake up on a waitqueue that this function registers for. However there are two races in the existing code 1) If space became available between the caller noticing there was no space and this function actually sleeping, the wakeup is missed and the timeout condition will happen instead 2) If a wakeup happened but not sufficient space became available, the code will loop again and wait for more space. However, if the second wake comes in prior to hitting the schedule_timeout_interruptible(), it will be missed, and potentially you'll wait out until the timeout happens. The fix consists of using more careful setting of the current state (so that if a wakeup happens in the main loop window, the schedule_timeout() falls through) and by checking for available space prior to going into the schedule_timeout() loop, but after being on the waitqueue and having the state set to interruptible. Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> Cc: Takashi Iwai <tiwai@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- sound/core/pcm_lib.c | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) diff -puN sound/core/pcm_lib.c~sound-core-pcm_libc-fix-race-condition-in-wait_for_avail sound/core/pcm_lib.c --- a/sound/core/pcm_lib.c~sound-core-pcm_libc-fix-race-condition-in-wait_for_avail +++ a/sound/core/pcm_lib.c @@ -1761,6 +1761,10 @@ static int wait_for_avail(struct snd_pcm snd_pcm_uframes_t avail = 0; long wait_time, tout; + init_waitqueue_entry(&wait, current); + add_wait_queue(&runtime->tsleep, &wait); + set_current_state(TASK_INTERRUPTIBLE); + if (runtime->no_period_wakeup) wait_time = MAX_SCHEDULE_TIMEOUT; else { @@ -1771,16 +1775,34 @@ static int wait_for_avail(struct snd_pcm } wait_time = msecs_to_jiffies(wait_time * 1000); } - init_waitqueue_entry(&wait, current); - add_wait_queue(&runtime->tsleep, &wait); + + /* + * We need to check if space became available already (and thus the + * wakeup happened already) prior to going into the sleep loop to + * close the race of space already having become available. + * This check must happen after been added to the waitqueue and + * having current state be INTERRUPTIBLE. + */ + + if (is_playback) + avail = snd_pcm_playback_avail(runtime); + else + avail = snd_pcm_capture_avail(runtime); + if (avail >= runtime->twake) + goto _endloop; + + for (;;) { if (signal_pending(current)) { err = -ERESTARTSYS; break; } snd_pcm_stream_unlock_irq(substream); - tout = schedule_timeout_interruptible(wait_time); + + tout = schedule_timeout(wait_time); + snd_pcm_stream_lock_irq(substream); + set_current_state(TASK_INTERRUPTIBLE); switch (runtime->status->state) { case SNDRV_PCM_STATE_SUSPENDED: err = -ESTRPIPE; @@ -1814,6 +1836,7 @@ static int wait_for_avail(struct snd_pcm break; } _endloop: + set_current_state(TASK_RUNNING); remove_wait_queue(&runtime->tsleep, &wait); *availp = avail; return err; _ Patches currently in -mm which might be from arjan@xxxxxxxxxxxxx are sound-core-pcm_libc-fix-race-condition-in-wait_for_avail.patch sound-core-pcm_libc-fix-race-condition-in-wait_for_avail-fix.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html