[PATCH 2/2] ALSA: seq: avoid stale FIFO cells during resize

[Cen Zhang <zzzccc427@gmail.com>]

snd_seq_fifo_resize() still needs to publish the replacement pool
before it waits for FIFO users. A blocking snd_seq_read() holds
f->use_lock while it sleeps, so concurrent senders must be able to
queue to the new pool and wake that reader instead of failing against a
closing old pool.

However, snd_seq_fifo_event_in() duplicates an event before it takes
f->lock, and snd_seq_read() can dequeue a cell and later call
snd_seq_fifo_cell_putback() if copy_to_user() or
snd_seq_expand_var_event() fails. If resize swaps f->pool and detaches
oldhead in between, either path can relink an old-pool cell after the
snapshot. That stale cell sits outside the drained oldhead list, keeps
oldpool->counter elevated, and can leave snd_seq_pool_delete() waiting
for the retired pool to drain.

Keep the existing swap-before-wait ordering in snd_seq_fifo_resize(),
but reject stale cells before any FIFO relink. Revalidate event-in cells
under f->lock and retry them against the published replacement pool, and
free stale putback cells instead of linking them back into the FIFO.

The buggy scenario involves two paths, with each column showing the
order within that path:

resize path:                    relink path:
1. Allocate newpool.             1. Take f->use_lock.
2. Swap f->pool to newpool and   2. Duplicate or dequeue an old-pool
   detach oldhead.                  cell before oldpool closes.
3. Mark oldpool closing and      3. Reach a later relink point after
   wait for FIFO users.             resize published newpool.
4. Free oldhead and delete       4. Relink the old-pool cell after
   oldpool.                         resize detached oldhead.
                                 5. Drop f->use_lock.

Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
 sound/core/seq/seq_fifo.c | 52 ++++++++++++++++++++++++++++-----------
 1 file changed, 37 insertions(+), 15 deletions(-)

diff --git a/sound/core/seq/seq_fifo.c b/sound/core/seq/seq_fifo.c
index ebe1394c18a9..cffa8d430374 100644
--- a/sound/core/seq/seq_fifo.c
+++ b/sound/core/seq/seq_fifo.c
@@ -101,13 +101,17 @@ int snd_seq_fifo_event_in(struct snd_seq_fifo *f,
 			  struct snd_seq_event *event)
 {
 	struct snd_seq_event_cell *cell;
+	struct snd_seq_pool *pool;
+	bool linked;
 	int err;
 
 	if (snd_BUG_ON(!f))
 		return -EINVAL;
 
 	guard(snd_seq_fifo)(f);
-	err = snd_seq_event_dup(f->pool, event, &cell, 1, NULL, NULL); /* always non-blocking */
+retry:
+	pool = READ_ONCE(f->pool);
+	err = snd_seq_event_dup(pool, event, &cell, 1, NULL, NULL); /* always non-blocking */
 	if (err < 0) {
 		if ((err == -ENOMEM) || (err == -EAGAIN))
 			atomic_inc(&f->overflow);
@@ -115,14 +119,24 @@ int snd_seq_fifo_event_in(struct snd_seq_fifo *f,
 	}
 		
 	/* append new cells to fifo */
+	linked = false;
 	scoped_guard(spinlock_irqsave, &f->lock) {
-		if (f->tail != NULL)
-			f->tail->next = cell;
-		f->tail = cell;
-		if (f->head == NULL)
-			f->head = cell;
-		cell->next = NULL;
-		f->cells++;
+		if (cell->pool == f->pool) {
+			if (f->tail)
+				f->tail->next = cell;
+			f->tail = cell;
+			if (!f->head)
+				f->head = cell;
+			cell->next = NULL;
+			f->cells++;
+			linked = true;
+		}
+	}
+
+	if (!linked) {
+		/* Retry against the replacement pool after resize publishes it. */
+		snd_seq_cell_free(cell);
+		goto retry;
 	}
 
 	/* wakeup client */
@@ -194,13 +208,21 @@ int snd_seq_fifo_cell_out(struct snd_seq_fifo *f,
 void snd_seq_fifo_cell_putback(struct snd_seq_fifo *f,
 			       struct snd_seq_event_cell *cell)
 {
+	bool linked = false;
+
 	if (cell) {
-		guard(spinlock_irqsave)(&f->lock);
-		cell->next = f->head;
-		f->head = cell;
-		if (!f->tail)
-			f->tail = cell;
-		f->cells++;
+		scoped_guard(spinlock_irqsave, &f->lock) {
+			if (cell->pool == f->pool) {
+				cell->next = f->head;
+				f->head = cell;
+				if (!f->tail)
+					f->tail = cell;
+				f->cells++;
+				linked = true;
+			}
+		}
+		if (!linked)
+			snd_seq_cell_free(cell);
 	}
 }
 
@@ -237,7 +259,7 @@ int snd_seq_fifo_resize(struct snd_seq_fifo *f, int poolsize)
 		oldpool = f->pool;
 		oldhead = f->head;
 		/* exchange pools */
-		f->pool = newpool;
+		WRITE_ONCE(f->pool, newpool);
 		f->head = NULL;
 		f->tail = NULL;
 		f->cells = 0;
-- 
2.43.0