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ALSA: seq: Use RCU for the virmidi file list

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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
Download Patch File Download Diff File Expand all files Collapse all files

1
include/sound/seq_virmidi.h
View File

@@ -46,7 +46,6 @@ struct snd_virmidi_dev {
int client; /* created/attached client */
int port; /* created/attached port */
unsigned int flags; /* SNDRV_VIRMIDI_* */
rwlock_t filelist_lock;
struct rw_semaphore filelist_sem;
struct list_head filelist;
};

19
sound/core/seq/seq_virmidi.c
View File

@@ -78,10 +78,11 @@ static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
int len;
if (atomic)
read_lock(&rdev->filelist_lock);
rcu_read_lock();
else
down_read(&rdev->filelist_sem);
list_for_each_entry(vmidi, &rdev->filelist, list) {
list_for_each_entry_rcu(vmidi, &rdev->filelist, list,
lockdep_is_held(&rdev->filelist_sem)) {
if (!READ_ONCE(vmidi->trigger))
continue;
if (ev->type == SNDRV_SEQ_EVENT_SYSEX) {
@@ -96,7 +97,7 @@ static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
}
}
if (atomic)
read_unlock(&rdev->filelist_lock);
rcu_read_unlock();
else
up_read(&rdev->filelist_sem);
@@ -200,8 +201,7 @@ static int snd_virmidi_input_open(struct snd_rawmidi_substream *substream)
vmidi->port = rdev->port;
runtime->private_data = vmidi;
scoped_guard(rwsem_write, &rdev->filelist_sem) {
guard(write_lock_irq)(&rdev->filelist_lock);
list_add_tail(&vmidi->list, &rdev->filelist);
list_add_tail_rcu(&vmidi->list, &rdev->filelist);
}
vmidi->rdev = rdev;
return 0;
@@ -243,9 +243,13 @@ static int snd_virmidi_input_close(struct snd_rawmidi_substream *substream)
struct snd_virmidi *vmidi = substream->runtime->private_data;
scoped_guard(rwsem_write, &rdev->filelist_sem) {
guard(write_lock_irq)(&rdev->filelist_lock);
list_del(&vmidi->list);
list_del_rcu(&vmidi->list);
}
/* wait for a grace period so that lockless readers in the atomic
* delivery path (snd_virmidi_dev_receive_event()) are no longer
* traversing this entry before its parser and memory are freed
*/
synchronize_rcu();
snd_midi_event_free(vmidi->parser);
substream->runtime->private_data = NULL;
kfree(vmidi);
@@ -508,7 +512,6 @@ int snd_virmidi_new(struct snd_card *card, int device, struct snd_rawmidi **rrmi
rdev->device = device;
rdev->client = -1;
init_rwsem(&rdev->filelist_sem);
rwlock_init(&rdev->filelist_lock);
INIT_LIST_HEAD(&rdev->filelist);
rdev->seq_mode = SNDRV_VIRMIDI_SEQ_DISPATCH;
rmidi->private_data = rdev;