[syzbot] [block?] possible deadlock in elv_iosched_store

[syzbot] [block?] possible deadlock in elv_iosched_store

[syzbot]

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@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.14.0-syzkaller-05877-g1a9239bb4253 #0 Not tainted
------------------------------------------------------
syz-executor192/5823 is trying to acquire lock:
ffff888144302cd8 (&q->elevator_lock){+.+.}-{4:4}, at: elv_iosched_store+0x201/0x5f0 block/elevator.c:735

but task is already holding lock:
ffff8881443027a8 (&q->q_usage_counter(io)#58){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#58){++++}-{0:0}:
       blk_alloc_queue+0x619/0x760 block/blk-core.c:461
       blk_mq_alloc_queue+0x179/0x290 block/blk-mq.c:4349
       __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4396
       nbd_dev_add+0x49d/0xbb0 drivers/block/nbd.c:1933
       nbd_init+0x181/0x320 drivers/block/nbd.c:2670
       do_one_initcall+0x120/0x6e0 init/main.c:1257
       do_initcall_level init/main.c:1319 [inline]
       do_initcalls init/main.c:1335 [inline]
       do_basic_setup init/main.c:1354 [inline]
       kernel_init_freeable+0x5c2/0x900 init/main.c:1567
       kernel_init+0x1c/0x2b0 init/main.c:1457
       ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (fs_reclaim){+.+.}-{0:0}:
       __fs_reclaim_acquire mm/page_alloc.c:3853 [inline]
       fs_reclaim_acquire+0x102/0x150 mm/page_alloc.c:3867
       might_alloc include/linux/sched/mm.h:318 [inline]
       xa_insert include/linux/xarray.h:783 [inline]
       blk_mq_init_hctx block/blk-mq.c:3924 [inline]
       blk_mq_alloc_and_init_hctx+0x503/0x11c0 block/blk-mq.c:4457
       blk_mq_realloc_hw_ctxs+0x8f6/0xc00 block/blk-mq.c:4486
       blk_mq_init_allocated_queue+0x3af/0x1230 block/blk-mq.c:4540
       blk_mq_alloc_queue+0x1c2/0x290 block/blk-mq.c:4353
       __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4396
       loop_add+0x496/0xb70 drivers/block/loop.c:2067
       loop_init+0x164/0x270 drivers/block/loop.c:2302
       do_one_initcall+0x120/0x6e0 init/main.c:1257
       do_initcall_level init/main.c:1319 [inline]
       do_initcalls init/main.c:1335 [inline]
       do_basic_setup init/main.c:1354 [inline]
       kernel_init_freeable+0x5c2/0x900 init/main.c:1567
       kernel_init+0x1c/0x2b0 init/main.c:1457
       ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (&q->elevator_lock){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3166 [inline]
       check_prevs_add kernel/locking/lockdep.c:3285 [inline]
       validate_chain kernel/locking/lockdep.c:3909 [inline]
       __lock_acquire+0x1173/0x1ba0 kernel/locking/lockdep.c:5235
       lock_acquire kernel/locking/lockdep.c:5866 [inline]
       lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5823
       __mutex_lock_common kernel/locking/mutex.c:587 [inline]
       __mutex_lock+0x19a/0xb00 kernel/locking/mutex.c:732
       elv_iosched_store+0x201/0x5f0 block/elevator.c:735
       queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
       sysfs_kf_write+0x117/0x170 fs/sysfs/file.c:139
       kernfs_fop_write_iter+0x349/0x510 fs/kernfs/file.c:334
       iter_file_splice_write+0x91c/0x1150 fs/splice.c:738
       do_splice_from fs/splice.c:935 [inline]
       direct_splice_actor+0x18f/0x6c0 fs/splice.c:1158
       splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
       do_splice_direct_actor fs/splice.c:1201 [inline]
       do_splice_direct+0x174/0x240 fs/splice.c:1227
       do_sendfile+0xafd/0xe50 fs/read_write.c:1368
       __do_sys_sendfile64 fs/read_write.c:1429 [inline]
       __se_sys_sendfile64 fs/read_write.c:1415 [inline]
       __x64_sys_sendfile64+0x1d8/0x220 fs/read_write.c:1415
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0x260 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &q->elevator_lock --> fs_reclaim --> &q->q_usage_counter(io)#58

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->q_usage_counter(io)#58);
                               lock(fs_reclaim);
                               lock(&q->q_usage_counter(io)#58);
  lock(&q->elevator_lock);

 *** DEADLOCK ***

5 locks held by syz-executor192/5823:
 #0: ffff888035bd2420 (sb_writers#7){.+.+}-{0:0}, at: splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
 #1: ffff888033e17088 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x287/0x510 fs/kernfs/file.c:325
 #2: ffff8880258933c8 (kn->active#47){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2aa/0x510 fs/kernfs/file.c:326
 #3: ffff8881443027a8 (&q->q_usage_counter(io)#58){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215
 #4: ffff8881443027e0 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x15/0x20 block/blk-mq.c:215

stack backtrace:
CPU: 0 UID: 0 PID: 5823 Comm: syz-executor192 Not tainted 6.14.0-syzkaller-05877-g1a9239bb4253 #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_circular_bug+0x275/0x350 kernel/locking/lockdep.c:2079
 check_noncircular+0x14c/0x170 kernel/locking/lockdep.c:2211
 check_prev_add kernel/locking/lockdep.c:3166 [inline]
 check_prevs_add kernel/locking/lockdep.c:3285 [inline]
 validate_chain kernel/locking/lockdep.c:3909 [inline]
 __lock_acquire+0x1173/0x1ba0 kernel/locking/lockdep.c:5235
 lock_acquire kernel/locking/lockdep.c:5866 [inline]
 lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5823
 __mutex_lock_common kernel/locking/mutex.c:587 [inline]
 __mutex_lock+0x19a/0xb00 kernel/locking/mutex.c:732
 elv_iosched_store+0x201/0x5f0 block/elevator.c:735
 queue_attr_store+0x270/0x310 block/blk-sysfs.c:799
 sysfs_kf_write+0x117/0x170 fs/sysfs/file.c:139
 kernfs_fop_write_iter+0x349/0x510 fs/kernfs/file.c:334
 iter_file_splice_write+0x91c/0x1150 fs/splice.c:738
 do_splice_from fs/splice.c:935 [inline]
 direct_splice_actor+0x18f/0x6c0 fs/splice.c:1158
 splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
 do_splice_direct_actor fs/splice.c:1201 [inline]
 do_splice_direct+0x174/0x240 fs/splice.c:1227
 do_sendfile+0xafd/0xe50 fs/read_write.c:1368
 __do_sys_sendfile64 fs/read_write.c:1429 [inline]
 __se_sys_sendfile64 fs/read_write.c:1415 [inline]
 __x64_sys_sendfile64+0x1d8/0x220 fs/read_write.c:1415
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x260 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff2c3abb2e9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fffc4e88b88 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00007fffc4e88d58 RCX: 00007ff2c3abb2e9
RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000003
RBP: 00007ff2c3b2e610 R08: 0000000000000000 R09: 00007fffc4e88d58
R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fffc4e88d48 R14: 0000000000000001 R15: 0000


---
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If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
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If you want to overwrite report's subsystems, reply with:
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If the report is a duplicate of another one, reply with:
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If you want to undo deduplication, reply with:
#syz undup

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@syzkaller.appspotmail.com
> 

...

> 
> 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

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

[Nilay Shroff]

On 3/29/25 7:29 AM, Ming Lei wrote:
> 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@syzkaller.appspotmail.com
>>
> 
> ...
> 
>>
>> 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;
> 

This patch looks good to me, however after we fix this one, I found another splat. 
I see that these new splats are side effect of commit ffa1e7ada456 ("block: Make 
request_queue lockdep splats show up earlier").

IMO in the block layer code (unless it's in an IO submission path or a path where we 
have already frozen queue) we may still want to allow memory allocation with GFP_KERNEL. 
So in that sense, for example, we may acquire ->elevator_lock followed by fs_reclaim. 
Or in another words, shouldn't it be legitimate to acquire blk layer specific lock and 
then allocate memory using GFP_KERNEL assuming we haven't freezed queue or we're not in 
IO submission path. But this commit ffa1e7ada456 ("block: Make request_queue lockdep
splats show up earlier") now showing up some false-positive splat as well, please see
below:

======================================================
WARNING: possible circular locking dependency detected
6.14.0+ #147 Not tainted
------------------------------------------------------
bash/5903 is trying to acquire lock:
c0000000ba0c6ad8 (&q->elevator_lock){+.+.}-{4:4}, at: elv_iosched_store+0x11c/0x5d4

but task is already holding lock:
c0000000ba0c65b8 (&q->q_usage_counter(io)#20){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#20){++++}-{0:0}:
       blk_alloc_queue+0x3a8/0x3e4
       blk_mq_alloc_queue+0x88/0x11c
       __blk_mq_alloc_disk+0x34/0xd8
       null_add_dev+0x3c8/0x914 [null_blk]
       null_init+0x1e0/0x4bc [null_blk]
       do_one_initcall+0x8c/0x4b8
       do_init_module+0x7c/0x2c4
       init_module_from_file+0xb4/0x108
       idempotent_init_module+0x26c/0x368
       sys_finit_module+0x98/0x150
       system_call_exception+0x134/0x360
       system_call_vectored_common+0x15c/0x2ec

-> #1 (fs_reclaim){+.+.}-{0:0}:
       fs_reclaim_acquire+0xe4/0x120
       kmem_cache_alloc_noprof+0x74/0x570
       __kernfs_new_node+0x98/0x378
       kernfs_new_node+0x80/0xc4
       kernfs_create_dir_ns+0x44/0xec
       sysfs_create_dir_ns+0x94/0x160
       kobject_add_internal+0xf4/0x3c8
       kobject_add+0x70/0x10c
       elv_register_queue+0x70/0x14c
       blk_register_queue+0x1d8/0x2bc
       add_disk_fwnode+0x3b4/0x5d0
       sd_probe+0x3bc/0x5b4 [sd_mod]
       really_probe+0x104/0x4c4
       __driver_probe_device+0xb8/0x200
       driver_probe_device+0x54/0x128
       __driver_attach_async_helper+0x7c/0x150
       async_run_entry_fn+0x60/0x1bc
       process_one_work+0x2ac/0x7e4
       worker_thread+0x238/0x460
       kthread+0x158/0x188
       start_kernel_thread+0x14/0x18

-> #0 (&q->elevator_lock){+.+.}-{4:4}:
       __lock_acquire+0x1b6c/0x2ae0
       lock_acquire+0x140/0x430
       __mutex_lock+0xf0/0xb00
       elv_iosched_store+0x11c/0x5d4
       queue_attr_store+0x12c/0x164
       sysfs_kf_write+0x6c/0xb0
       kernfs_fop_write_iter+0x1ac/0x2a8
       vfs_write+0x410/0x584
       ksys_write+0x84/0x140
       system_call_exception+0x134/0x360
       system_call_vectored_common+0x15c/0x2ec

other info that might help us debug this:

Chain exists of:
  &q->elevator_lock --> fs_reclaim --> &q->q_usage_counter(io)#20

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->q_usage_counter(io)#20);
                               lock(fs_reclaim);
                               lock(&q->q_usage_counter(io)#20);
  lock(&q->elevator_lock);

 *** DEADLOCK ***

5 locks held by bash/5903:
 #0: c00000005cb7f400 (sb_writers#3){.+.+}-{0:0}, at: ksys_write+0x84/0x140
 #1: c000000008711288 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x168/0x2a8
 #2: c00000000a1e2c08 (kn->active#57){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x174/0x2a8
 #3: c0000000ba0c65b8 (&q->q_usage_counter(io)#20){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40
 #4: c0000000ba0c65f0 (&q->q_usage_counter(queue)#21){+.+.}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40

stack backtrace:
CPU: 17 UID: 0 PID: 5903 Comm: bash Kdump: loaded Not tainted 6.14.0+ #147 VOLUNTARY 
Hardware name: IBM,9043-MRX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.00 (NM1060_028) hv:phyp pSeries
Call Trace:
[c0000000955df580] [c0000000011a7ef8] dump_stack_lvl+0x108/0x18c (unreliable)
[c0000000955df5b0] [c000000000225b0c] print_circular_bug+0x448/0x604
[c0000000955df660] [c000000000225f14] check_noncircular+0x24c/0x26c
[c0000000955df730] [c00000000022c3e8] __lock_acquire+0x1b6c/0x2ae0
[c0000000955df860] [c000000000229700] lock_acquire+0x140/0x430
[c0000000955df960] [c0000000011e84e8] __mutex_lock+0xf0/0xb00
[c0000000955dfa90] [c0000000008fb6f8] elv_iosched_store+0x11c/0x5d4
[c0000000955dfb50] [c000000000903ec0] queue_attr_store+0x12c/0x164
[c0000000955dfc60] [c0000000007ca58c] sysfs_kf_write+0x6c/0xb0
[c0000000955dfca0] [c0000000007c8df0] kernfs_fop_write_iter+0x1ac/0x2a8
[c0000000955dfcf0] [c0000000006a8c9c] vfs_write+0x410/0x584
[c0000000955dfdc0] [c0000000006a9148] ksys_write+0x84/0x140
[c0000000955dfe10] [c000000000031814] system_call_exception+0x134/0x360
[c0000000955dfe50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec
   
What do you think?

Thanks,
--Nilay

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

[Ming Lei]

On Tue, Apr 01, 2025 at 05:23:56PM +0530, Nilay Shroff wrote:
> 
> 
> On 3/29/25 7:29 AM, Ming Lei wrote:
> > 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@syzkaller.appspotmail.com
> >>
> > 
> > ...
> > 
> >>
> >> 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;
> > 
> 
> This patch looks good to me, however after we fix this one, I found another splat. 
> I see that these new splats are side effect of commit ffa1e7ada456 ("block: Make 
> request_queue lockdep splats show up earlier").
> 
> IMO in the block layer code (unless it's in an IO submission path or a path where we 
> have already frozen queue) we may still want to allow memory allocation with GFP_KERNEL. 
> So in that sense, for example, we may acquire ->elevator_lock followed by fs_reclaim. 

If any memory GFP_KERNEL allocation grabs ->elevator_lock, it is one real
deadlock risk.

> Or in another words, shouldn't it be legitimate to acquire blk layer specific lock and 
> then allocate memory using GFP_KERNEL assuming we haven't freezed queue or we're not in 
> IO submission path. But this commit ffa1e7ada456 ("block: Make request_queue lockdep
> splats show up earlier") now showing up some false-positive splat as well, please see
> below:

It depends if we may run GFP_KERNEL allocation with ->elevator_lock.

I feel ->elevator_lock is still used too many...


thanks,
Ming

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

[Nilay Shroff]

On 4/1/25 5:46 PM, Ming Lei wrote:
> On Tue, Apr 01, 2025 at 05:23:56PM +0530, Nilay Shroff wrote:
>>
>>
>> On 3/29/25 7:29 AM, Ming Lei wrote:
>>> 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@syzkaller.appspotmail.com
>>>>
>>>
>>> ...
>>>
>>>>
>>>> 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;
>>>
>>
>> This patch looks good to me, however after we fix this one, I found another splat. 
>> I see that these new splats are side effect of commit ffa1e7ada456 ("block: Make 
>> request_queue lockdep splats show up earlier").
>>
>> IMO in the block layer code (unless it's in an IO submission path or a path where we 
>> have already frozen queue) we may still want to allow memory allocation with GFP_KERNEL. 
>> So in that sense, for example, we may acquire ->elevator_lock followed by fs_reclaim. 
> 
> If any memory GFP_KERNEL allocation grabs ->elevator_lock, it is one real
> deadlock risk.
> 
>> Or in another words, shouldn't it be legitimate to acquire blk layer specific lock and 
>> then allocate memory using GFP_KERNEL assuming we haven't freezed queue or we're not in 
>> IO submission path. But this commit ffa1e7ada456 ("block: Make request_queue lockdep
>> splats show up earlier") now showing up some false-positive splat as well, please see
>> below:
> 
> It depends if we may run GFP_KERNEL allocation with ->elevator_lock.
> 
Okay, so do you think we shall use GFP_NOIO for memory allocation if it's done after we
acquire ->elevator_lock?

Thanks,
--Nilay

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@syzkaller.appspotmail.com
> 

...

> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> 
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> 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.
> 
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report

#syz dup: [syzbot] [block?] possible deadlock in elv_iosched_store

-- 
Ming

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

[syzbot]

> 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@syzkaller.appspotmail.com
>> 
>
> ...
>
>> 
>> ---
>> This report is generated by a bot. It may contain errors.
>> See https://goo.gl/tpsmEJ for more information about syzbot.
>> syzbot engineers can be reached at syzkaller@googlegroups.com.
>> 
>> syzbot will keep track of this issue. See:
>> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>> 
>> If the report is already addressed, let syzbot know by replying with:
>> #syz fix: exact-commit-title
>> 
>> 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.
>> 
>> If you want to overwrite report's subsystems, reply with:
>> #syz set subsystems: new-subsystem
>> (See the list of subsystem names on the web dashboard)
>> 
>> If the report is a duplicate of another one, reply with:
>> #syz dup: exact-subject-of-another-report
>
> #syz dup: [syzbot] [block?] possible deadlock in elv_iosched_store

Can't dup bug to itself.

>
> -- 
> Ming
>