[syzbot ci] Re: mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain

[syzbot ci <syzbot+ci3c65cfe4c9a15aea@syzkaller.appspotmail.com>]

syzbot ci has tested the following series

[v2] mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain
https://lore.kernel.org/all/20260330175824.2777270-1-urezki@gmail.com
* [PATCH v2] mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain

and found the following issue:
possible deadlock in touch_wq_lockdep_map

Full report is available here:
https://ci.syzbot.org/series/4a8f638e-3a3f-4346-8189-2b5a0eb9ceaf

***

possible deadlock in touch_wq_lockdep_map

tree:      mm-new
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/mm.git
base:      af42d6d3650b95295a3f08fc35189998bc26c2e1
arch:      amd64
compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config:    https://ci.syzbot.org/builds/d94b1f61-6a69-4415-b385-b3c99018e8c2/config
syz repro: https://ci.syzbot.org/findings/01e4d436-e924-4ce6-8902-78c6000bf34e/syz_repro

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
kworker/u9:1/33 is trying to acquire lock:
ffff88810168e948 ((wq_completion)vmap_drain){+.+.}-{0:0}, at: touch_wq_lockdep_map+0xb5/0x180 kernel/workqueue.c:3991

but task is already holding lock:
ffff88810168e948 ((wq_completion)vmap_drain){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3251 [inline]
ffff88810168e948 ((wq_completion)vmap_drain){+.+.}-{0:0}, at: process_scheduled_works+0xa52/0x18c0 kernel/workqueue.c:3359

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock((wq_completion)vmap_drain);
  lock((wq_completion)vmap_drain);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

4 locks held by kworker/u9:1/33:
 #0: ffff88810168e948 ((wq_completion)vmap_drain){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3251 [inline]
 #0: ffff88810168e948 ((wq_completion)vmap_drain){+.+.}-{0:0}, at: process_scheduled_works+0xa52/0x18c0 kernel/workqueue.c:3359
 #1: ffffc90000a97c40 (drain_vmap_work){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3252 [inline]
 #1: ffffc90000a97c40 (drain_vmap_work){+.+.}-{0:0}, at: process_scheduled_works+0xa8d/0x18c0 kernel/workqueue.c:3359
 #2: ffffffff8e87ed68 (vmap_purge_lock){+.+.}-{4:4}, at: drain_vmap_area_work+0x17/0x40 mm/vmalloc.c:2443
 #3: ffffffff8e75e5e0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:312 [inline]
 #3: ffffffff8e75e5e0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:850 [inline]
 #3: ffffffff8e75e5e0 (rcu_read_lock){....}-{1:3}, at: start_flush_work kernel/workqueue.c:4234 [inline]
 #3: ffffffff8e75e5e0 (rcu_read_lock){....}-{1:3}, at: __flush_work+0x100/0xc50 kernel/workqueue.c:4292

stack backtrace:
CPU: 1 UID: 0 PID: 33 Comm: kworker/u9:1 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Workqueue: vmap_drain drain_vmap_area_work
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain kernel/locking/lockdep.c:3895 [inline]
 __lock_acquire+0x253f/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
 touch_wq_lockdep_map+0xcb/0x180 kernel/workqueue.c:3991
 start_flush_work kernel/workqueue.c:4272 [inline]
 __flush_work+0x87c/0xc50 kernel/workqueue.c:4292
 __purge_vmap_area_lazy+0x7db/0xab0 mm/vmalloc.c:2419
 drain_vmap_area_work+0x27/0x40 mm/vmalloc.c:2444
 process_one_work kernel/workqueue.c:3276 [inline]
 process_scheduled_works+0xb6e/0x18c0 kernel/workqueue.c:3359
 worker_thread+0xa53/0xfc0 kernel/workqueue.c:3440
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

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