ALSA: seq: Use RCU for the virmidi file list

Browse source

Each virmidi device keeps a list of its opened input files (filelist)
protected by both an rwlock (filelist_lock) and a rw_semaphore
(filelist_sem).  snd_virmidi_dev_receive_event() walks the list on the
sequencer event input path -- read_lock() when the event is delivered in
atomic context, down_read() otherwise -- decoding each incoming event
into the file's rawmidi buffer.  The writers (input open/close) take both
locks to add/remove entries.

This is another typical dual-lock read-mostly pattern as the port
subscriber list: files are opened/closed rarely while the receive
callback runs per event.  Let's convert the traversal to RCU and drop
the rwlock; the existing filelist_sem keeps serializing the writers.
The atomic input path now runs lock-free under rcu_read_lock(), and
both readers share a single list_for_each_entry_rcu() (valid under the
rwsem via lockdep_is_held()).  The writers switch to
list_add_tail_rcu() / list_del_rcu().

snd_virmidi_input_close() freed the entry (parser and struct)
immediately after list_del.  A concurrent lockless reader in the atomic
path may still be dereferencing it, so the close path now waits for an
RCU grace period after list_del_rcu() before freeing; synchronize_rcu()
is used rather than kfree_rcu() because the parser must also be released
after the grace period, not just the struct.  Non-atomic readers are
already excluded by the down_write, so only the atomic RCU readers need
the grace period.

Dropping write_lock_irq() from the writers is safe: no writer runs in
atomic/IRQ context, and the sole atomic reader now uses RCU, which is
IRQ-safe.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260810133711.42483-5-tiwai@suse.de

...

This commit is contained in:

Takashi Iwai 2026-08-10 15:37:05 +02:00

parent 7a287e4615

commit 7ffa5d2462

2 changed files with 11 additions and 9 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

1

include/sound/seq_virmidi.h

View file

19

sound/core/seq/seq_virmidi.c

View file