Re: [syzbot] [block?] possible deadlock in elv_iosched_store

[Ming Lei]

On Fri, Mar 28, 2025 at 07:37:25AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1a9239bb4253 Merge tag 'net-next-6.15' of git://git.kernel..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1384b43f980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=c7163a109ac459a8
> dashboard link: https://syzkaller.appspot.com/bug?extid=4c7e0f9b94ad65811efb
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=178cfa4c580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11a8ca4c580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/fc7dc9f0d9a7/disk-1a9239bb.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f555a3ae03d3/vmlinux-1a9239bb.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/55f6ea74eaf2/bzImage-1a9239bb.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+4c7e0f9b94ad65811efb@xxxxxxxxxxxxxxxxxxxxxxxxx
> 

...

> 
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.


diff --git a/block/blk-mq.c b/block/blk-mq.c
index ae8494d88897..d7a103dc258b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4465,14 +4465,12 @@ static struct blk_mq_hw_ctx *blk_mq_alloc_and_init_hctx(
 	return NULL;
 }
 
-static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
-						struct request_queue *q)
+static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
+				     struct request_queue *q)
 {
 	struct blk_mq_hw_ctx *hctx;
 	unsigned long i, j;
 
-	/* protect against switching io scheduler  */
-	mutex_lock(&q->elevator_lock);
 	for (i = 0; i < set->nr_hw_queues; i++) {
 		int old_node;
 		int node = blk_mq_get_hctx_node(set, i);
@@ -4505,7 +4503,19 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
 
 	xa_for_each_start(&q->hctx_table, j, hctx, j)
 		blk_mq_exit_hctx(q, set, hctx, j);
-	mutex_unlock(&q->elevator_lock);
+}
+
+static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
+				   struct request_queue *q, bool lock)
+{
+	if (lock) {
+		/* protect against switching io scheduler  */
+		mutex_lock(&q->elevator_lock);
+		__blk_mq_realloc_hw_ctxs(set, q);
+		mutex_unlock(&q->elevator_lock);
+	} else {
+		__blk_mq_realloc_hw_ctxs(set, q);
+	}
 
 	/* unregister cpuhp callbacks for exited hctxs */
 	blk_mq_remove_hw_queues_cpuhp(q);
@@ -4537,7 +4547,7 @@ int blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 
 	xa_init(&q->hctx_table);
 
-	blk_mq_realloc_hw_ctxs(set, q);
+	blk_mq_realloc_hw_ctxs(set, q, false);
 	if (!q->nr_hw_queues)
 		goto err_hctxs;
 
@@ -5033,7 +5043,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 fallback:
 	blk_mq_update_queue_map(set);
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
-		blk_mq_realloc_hw_ctxs(set, q);
+		blk_mq_realloc_hw_ctxs(set, q, true);
 
 		if (q->nr_hw_queues != set->nr_hw_queues) {
 			int i = prev_nr_hw_queues;

Thanks,
Ming