[alsa-devel] [RFC][PATCH] ALSA: firewire-lib: permit process context only to flush queued packets for better PCM period granularity

[Takashi Iwai]

On Tue, 10 May 2016 16:07:50 +0200,
Takashi Sakamoto wrote:
> 
> Hi Clemens,
> 
> On 2016年05月07日 21:46, Takashi Sakamoto wrote:
> > Could I request your comment to this patch? It is to solve a race condition,
> > but the condition is quite rare. Practically, it might have a less meanings,
> > except for better program.
> > 
> > And I think there's another race condition against processing each packets
> > by calling out/in_stream_callback(), but I cannot observe the race. Software
> > IRQ contexts of IR/IT contexts and process contexts are under the race
> > condition, however I can see no problems related to it in my several trials
> > in multi-core machine. I have no idea about the reason that packet sequence
> > is processed correctly between the software IRQ contexts and the process
> > contexts without any lock primitives. Do you have some ideas about it?
> 
> I wrote an additional patch for this race issue. Would you please read
> this, too?

It's just a flag indicating of a busy task, right?
If so, it doesn't have to be a spinlock, but a simple atomic_t.


thanks,

Takashi

> 
> 
> Regards
> 
> ----- 8< -----
> 
> From d2090cac868e718227596dbda31ea6333b72009c Mon Sep 17 00:00:00 2001
> From: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> Date: Tue, 10 May 2016 22:02:05 +0900
> Subject: [PATCH] firewire-lib: add locking for packet processing
> 
> When packet streaming starts, packet processing is done in software IRQ
> context of 1394 OHCI IR/IT contexts. This is a typical way. On the other
> hand, process context of PCM application can also process packets in a
> path to handle PCM frames. This is for better PCM pointer granularity.
> The two execution context causes race condition against packet processing.
> 
> When the race occurs, it's enough that just one of these two contexts
> handles packet processing, because actual time dominates packet queueing.
> 
> This commit adds spin lock to manage the race condition. When the race
> occurs, second context returns immediately from critical section. Thus,
> it has little overhead.
> ---
>  sound/firewire/amdtp-stream.c | 39 ++++++++++++++++++++++++++++++---------
>  sound/firewire/amdtp-stream.h |  3 +++
>  2 files changed, 33 insertions(+), 9 deletions(-)
> 
> diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
> index 92d5a16..80d5887 100644
> --- a/sound/firewire/amdtp-stream.c
> +++ b/sound/firewire/amdtp-stream.c
> @@ -601,6 +601,14 @@ static void out_stream_callback(struct
> fw_iso_context *context, u32 tstamp,
>  	if (s->packet_index < 0)
>  		return;
> 
> +	/*
> +	 * It's enough for queued packets to be handled by process context of
> +	 * PCM application or software IRQ context of 1394 OHCI IT context in a
> +	 * time.
> +	 */
> +	if (!spin_trylock(&s->lock_packetization))
> +		return;
> +
>  	cycle = compute_cycle_count(tstamp);
> 
>  	/* Align to actual cycle count for the last packet. */
> @@ -608,14 +616,18 @@ static void out_stream_callback(struct
> fw_iso_context *context, u32 tstamp,
> 
>  	for (i = 0; i < packets; ++i) {
>  		cycle = increment_cycle_count(cycle, 1);
> -		if (handle_out_packet(s, cycle, i) < 0) {
> -			s->packet_index = -1;
> -			amdtp_stream_pcm_abort(s);
> -			return;
> -		}
> +		if (handle_out_packet(s, cycle, i) < 0)
> +			break;
>  	}
> 
> -	fw_iso_context_queue_flush(s->context);
> +	if (i == packets) {
> +		fw_iso_context_queue_flush(s->context);
> +	} else {
> +		s->packet_index = -1;
> +		amdtp_stream_pcm_abort(s);
> +	}
> +
> +	spin_unlock(&s->lock_packetization);
>  }
> 
>  static void in_stream_callback(struct fw_iso_context *context, u32 tstamp,
> @@ -631,6 +643,14 @@ static void in_stream_callback(struct
> fw_iso_context *context, u32 tstamp,
>  	if (s->packet_index < 0)
>  		return;
> 
> +	/*
> +	 * It's enough for queued packets to be handled by process context of
> +	 * PCM application or software IRQ context of 1394 OHCI IR context in a
> +	 * time.
> +	 */
> +	if (!spin_trylock(&s->lock_packetization))
> +		return;
> +
>  	/* The number of packets in buffer */
>  	packets = header_length / IN_PACKET_HEADER_SIZE;
> 
> @@ -660,13 +680,14 @@ static void in_stream_callback(struct
> fw_iso_context *context, u32 tstamp,
>  	}
> 
>  	/* Queueing error or detecting invalid payload. */
> -	if (i < packets) {
> +	if (i == packets) {
> +		fw_iso_context_queue_flush(s->context);
> +	} else {
>  		s->packet_index = -1;
>  		amdtp_stream_pcm_abort(s);
> -		return;
>  	}
> 
> -	fw_iso_context_queue_flush(s->context);
> +	spin_unlock(&s->lock_packetization);
>  }
> 
>  /* this is executed one time */
> diff --git a/sound/firewire/amdtp-stream.h b/sound/firewire/amdtp-stream.h
> index c1bc7fa..6be5feb 100644
> --- a/sound/firewire/amdtp-stream.h
> +++ b/sound/firewire/amdtp-stream.h
> @@ -102,6 +102,9 @@ struct amdtp_stream {
>  	struct iso_packets_buffer buffer;
>  	int packet_index;
> 
> +	/* Packet processing can run in one context at a time. */
> +	spinlock_t lock_packetization;
> +
>  	/* For CIP headers. */
>  	unsigned int source_node_id_field;
>  	unsigned int data_block_quadlets;
> -- 
> 2.7.4
> _______________________________________________
> Alsa-devel mailing list
> Alsa-devel at alsa-project.org
> http://mailman.alsa-project.org/mailman/listinfo/alsa-devel