ALSA: seq: midi: Optimize event_input locking with RCU

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: ef7607ab1c8a ("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>

0 files changed, 0 insertions, 0 deletions