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ALSA: seq: midi: Optimize event_input locking with RCU

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The recent fix for serializing the output teardown introduced a
spinlock invocation at every MIDI output event via event_process_midi.
Since this is a hot path, let's do performance optimization with RCU.

The new output_substream __rcu pointer is published via
rcu_assign_pointer() in midisynth_use() after output_rfile is set, and
cleared in midisynth_unuse() before the resource teardown.
event_process_midi() reads it under rcu_read_lock() and bumps
output_use_lock inside that section, which is necessary to close the
window between the pointer dereference and the refcount increment.

midisynth_unuse() calls synchronize_rcu() before snd_use_lock_sync():
this guarantees that any reader who obtained a non-NULL pointer has
already called atomic_inc (output_use_lock), so the subsequent
snd_use_lock_sync() sees the correct in-flight count.

Fixes: ef7607ab1c ("ALSA: seq: midi: Serialize output teardown with event_input")
Link: https://patch.msgid.link/20260813144224.753399-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Iwai 2026-08-13 16:42:16 +02:00
parent 0c4ef23e66
commit 4cc25cdd3c
1 changed files with 11 additions and 13 deletions
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24
sound/core/seq/seq_midi.c
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@ -43,8 +43,8 @@ struct seq_midisynth {
int device;
int subdevice;
struct snd_rawmidi_file input_rfile;
spinlock_t output_lock; /* protects output_rfile publication */
snd_use_lock_t output_use_lock; /* in-flight event_input users */
struct snd_rawmidi_substream __rcu *output_substream;
struct snd_rawmidi_file output_rfile;
int seq_client;
int seq_port;
@ -134,8 +134,8 @@ static int event_process_midi(struct snd_seq_event *ev, int direct,
if (snd_BUG_ON(!msynth))
return -EINVAL;
scoped_guard(spinlock_irqsave, &msynth->output_lock) {
substream = msynth->output_rfile.output;
scoped_guard(rcu) {
substream = rcu_dereference(msynth->output_substream);
if (!substream)
return -ENODEV;
snd_use_lock_use(&msynth->output_use_lock);
@ -177,7 +177,6 @@ static int snd_seq_midisynth_new(struct seq_midisynth *msynth,
msynth->card = card;
msynth->device = device;
msynth->subdevice = subdevice;
spin_lock_init(&msynth->output_lock);
snd_use_lock_init(&msynth->output_use_lock);
return 0;
}
@ -252,8 +251,8 @@ static int midisynth_use(void *private_data, struct snd_seq_port_subscribe *info
return err;
}
snd_midi_event_reset_decode(msynth->parser);
scoped_guard(spinlock_irqsave, &msynth->output_lock)
msynth->output_rfile = rfile;
msynth->output_rfile = rfile;
rcu_assign_pointer(msynth->output_substream, rfile.output);
return 0;
}
@ -261,17 +260,16 @@ static int midisynth_use(void *private_data, struct snd_seq_port_subscribe *info
static int midisynth_unuse(void *private_data, struct snd_seq_port_subscribe *info)
{
struct seq_midisynth *msynth = private_data;
struct snd_rawmidi_file rfile = {};
struct snd_rawmidi_file rfile;
scoped_guard(spinlock_irqsave, &msynth->output_lock) {
rfile = msynth->output_rfile;
msynth->output_rfile = (struct snd_rawmidi_file){};
}
rcu_assign_pointer(msynth->output_substream, NULL);
synchronize_rcu();
snd_use_lock_sync(&msynth->output_use_lock);
rfile = msynth->output_rfile;
msynth->output_rfile = (struct snd_rawmidi_file){};
if (snd_BUG_ON(!rfile.output))
return -EINVAL;
snd_use_lock_sync(&msynth->output_use_lock);
snd_rawmidi_drain_output(rfile.output);
return snd_rawmidi_kernel_release(&rfile);
}