On Wed, 11 May 2016 00:56:39 +0200, Takashi Sakamoto wrote: > > Hi, > > On May 11 2016 00:03, Takashi Iwai wrote: > > 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. > > This function is called in both of software IRQ context and process > context. Thus, atomic_t causes kernel hungs in software IRQ context, > because We cannot call kernel APIs which call process scheduler in the > context. I guess you are confused. atomic ops can be safely used in any contexts. (I suggested atomic_t in the previous mail, but actually a lighter primitive for such a case is cmpxchg() and its variants. In anyway, all are found in atomic.h.) In your code, you never call spin_lock() but only spin_trylock(). That is, there is no part really spinning, but it's used only as a flag. So, it is equivalent with doing something like: if (cmpxchg(flag, 1)) /* concurrently running? */ return; do_something; cmpxchg(flag, 0); in both callbacks. Takashi > For supplemental information, please refer to a patch which I posted > just now. Unfortunately, alsa-project.org doesn't blast the message as > of now... You can also see the message in an archive of ffado-devel. > https://sourceforge.net/p/ffado/mailman/message/35078341/ > > > Thanks > > Takashi Sakamoto > > > thanks, > > > > Takashi > > > >> > >> > >> Regards > >> > >> ----- 8< ----- > >> > >> From d2090cac868e718227596dbda31ea6333b72009c Mon Sep 17 00:00:00 2001 > >> From: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx> > >> 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@xxxxxxxxxxxxxxxx > >> http://mailman.alsa-project.org/mailman/listinfo/alsa-devel > > > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
