Hello Takashi, > -----Original Message----- > From: Takashi Iwai [mailto:tiwai@xxxxxxx] > Sent: Wednesday, November 20, 2019 6:32 PM > To: Gabbasov, Andrew > Cc: alsa-devel@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Jaroslav > Kysela; Takashi Iwai; Timo Wischer > Subject: Re: [PATCH v4 6/7] ALSA: aloop: Support selection of snd_timer > instead of jiffies > > On Wed, 20 Nov 2019 16:21:36 +0100, > Andrew Gabbasov wrote: > > > > Hello Takashi, > > > > > -----Original Message----- > > > From: Takashi Iwai [mailto:tiwai@xxxxxxx] > > > Sent: Wednesday, November 20, 2019 5:34 PM > > > To: Gabbasov, Andrew > > > Cc: alsa-devel@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Jaroslav > > > Kysela; Takashi Iwai; Timo Wischer > > > Subject: Re: [PATCH v4 6/7] ALSA: aloop: Support selection of snd_timer > > > instead of jiffies > > > > > > On Wed, 20 Nov 2019 12:58:55 +0100, > > > Andrew Gabbasov wrote: > > > > +/* call in loopback->cable_lock */ > > > > +static int loopback_snd_timer_open(struct loopback_pcm *dpcm) > > > > +{ > > > > + int err = 0; > > > > + struct snd_timer_id tid = { > > > > + .dev_class = SNDRV_TIMER_CLASS_PCM, > > > > + .dev_sclass = SNDRV_TIMER_SCLASS_APPLICATION, > > > > + }; > > > > + struct snd_timer_instance *timeri; > > > > + struct loopback_cable *cable = dpcm->cable; > > > > + > > > > + spin_lock_irq(&cable->lock); > > > > + > > > > + /* check if timer was already opened. It is only opened once > > > > + * per playback and capture subdevice (aka cable). > > > > + */ > > > > + if (cable->snd_timer.instance) > > > > + goto unlock; > > > > + > > > > + err = loopback_parse_timer_id(dpcm->loopback->timer_source, > &tid); > > > > + if (err < 0) { > > > > + pcm_err(dpcm->substream->pcm, > > > > + "Parsing timer source \'%s\' failed with %d", > > > > + dpcm->loopback->timer_source, err); > > > > + goto unlock; > > > > + } > > > > + > > > > + cable->snd_timer.stream = dpcm->substream->stream; > > > > + cable->snd_timer.id = tid; > > > > + > > > > + timeri = snd_timer_instance_new(dpcm->loopback->card->id); > > > > + if (!timeri) { > > > > + err = -ENOMEM; > > > > + goto unlock; > > > > + } > > > > + /* The callback has to be called from another tasklet. If > > > > + * SNDRV_TIMER_IFLG_FAST is specified it will be called from > the > > > > + * snd_pcm_period_elapsed() call of the selected sound card. > > > > + * snd_pcm_period_elapsed() helds > snd_pcm_stream_lock_irqsave(). > > > > + * Due to our callback loopback_snd_timer_function() also > calls > > > > + * snd_pcm_period_elapsed() which calls > > > snd_pcm_stream_lock_irqsave(). > > > > + * This would end up in a dead lock. > > > > + */ > > > > + timeri->flags |= SNDRV_TIMER_IFLG_AUTO; > > > > + timeri->callback = loopback_snd_timer_function; > > > > + timeri->callback_data = (void *)cable; > > > > + timeri->ccallback = loopback_snd_timer_event; > > > > + > > > > + /* snd_timer_close() and snd_timer_open() should not be called > with > > > > + * locked spinlock because both functions can block on a > mutex. The > > > > + * mutex loopback->cable_lock is kept locked. Therefore > > > snd_timer_open() > > > > + * cannot be called a second time by the other device of the > same > > > cable. > > > > + * Therefore the following issue cannot happen: > > > > + * [proc1] Call loopback_timer_open() -> > > > > + * Unlock cable->lock for snd_timer_close/open() call > > > > + * [proc2] Call loopback_timer_open() -> snd_timer_open(), > > > > + * snd_timer_start() > > > > + * [proc1] Call snd_timer_open() and overwrite running timer > > > > + * instance > > > > + */ > > > > + spin_unlock_irq(&cable->lock); > > > > + err = snd_timer_open(timeri, &cable->snd_timer.id, current- > >pid); > > > > + if (err < 0) { > > > > + pcm_err(dpcm->substream->pcm, > > > > + "snd_timer_open (%d,%d,%d) failed with %d", > > > > + cable->snd_timer.id.card, > > > > + cable->snd_timer.id.device, > > > > + cable->snd_timer.id.subdevice, > > > > + err); > > > > + snd_timer_instance_free(timeri); > > > > + return err; > > > > + } > > > > + spin_lock_irq(&cable->lock); > > > > + > > > > + cable->snd_timer.instance = timeri; > > > > + > > > > + /* initialise a tasklet used for draining */ > > > > + tasklet_init(&cable->snd_timer.event_tasklet, > > > > + loopback_snd_timer_tasklet, (unsigned > long)timeri); > > > > > > This has to be set before snd_timer_open(). The callback might be > > > called immediately after snd_timer_open(). > > > > This tasklet is used/scheduled only in ccallback (not regular tick > > callback), > > and only for SNDRV_TIMER_EVENT_MSTOP event. Can this event really happen > > immediately after snd_timer_open()? > > Why not? The master timer can be stopped at any time, even between > these two lines. > > Beware that there are fuzzer programs that can trigger such racy > things, and you're adding the code to the target that is actively > slapped by them :) OK, got it. I'll move this initialization to before snd_timer_open() in the next update together with the fixes for the other issues you will find in this version. Thanks! Best regards, Andrew _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx https://mailman.alsa-project.org/mailman/listinfo/alsa-devel
