[PATCH 5.15 169/177] io-wq: drop wqe lock before creating new worker

Greg Kroah-Hartman
2021-12-20T15:03:13Z

From: Jens Axboe <axboe@kernel.dk>

commit d800c65c2d4eccebb27ffb7808e842d5b533823c upstream.

We have two io-wq creation paths:

- On queue enqueue
- When a worker goes to sleep

The latter invokes worker creation with the wqe->lock held, but that can
run into problems if we end up exiting and need to cancel the queued work.
syzbot caught this:

============================================
WARNING: possible recursive locking detected
5.16.0-rc4-syzkaller #0 Not tainted
--------------------------------------------
iou-wrk-6468/6471 is trying to acquire lock:
ffff88801aa98018 (&wqe->lock){+.+.}-{2:2}, at: io_worker_cancel_cb+0xb7/0x210 fs/io-wq.c:187

but task is already holding lock:
ffff88801aa98018 (&wqe->lock){+.+.}-{2:2}, at: io_wq_worker_sleeping+0xb6/0x140 fs/io-wq.c:700

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

CPU0
----
lock(&wqe->lock);
lock(&wqe->lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

1 lock held by iou-wrk-6468/6471:
#0: ffff88801aa98018 (&wqe->lock){+.+.}-{2:2}, at: io_wq_worker_sleeping+0xb6/0x140 fs/io-wq.c:700

stack backtrace:
CPU: 1 PID: 6471 Comm: iou-wrk-6468 Not tainted 5.16.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1dc/0x2d8 lib/dump_stack.c:106
print_deadlock_bug kernel/locking/lockdep.c:2956 [inline]
check_deadlock kernel/locking/lockdep.c:2999 [inline]
validate_chain+0x5984/0x8240 kernel/locking/lockdep.c:3788
__lock_acquire+0x1382/0x2b00 kernel/locking/lockdep.c:5027
lock_acquire+0x19f/0x4d0 kernel/locking/lockdep.c:5637
__raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
_raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:154
io_worker_cancel_cb+0xb7/0x210 fs/io-wq.c:187
io_wq_cancel_tw_create fs/io-wq.c:1220 [inline]
io_queue_worker_create+0x3cf/0x4c0 fs/io-wq.c:372
io_wq_worker_sleeping+0xbe/0x140 fs/io-wq.c:701
sched_submit_work kernel/sched/core.c:6295 [inline]
schedule+0x67/0x1f0 kernel/sched/core.c:6323
schedule_timeout+0xac/0x300 kernel/time/timer.c:1857
wait_woken+0xca/0x1b0 kernel/sched/wait.c:460
unix_msg_wait_data net/unix/unix_bpf.c:32 [inline]
unix_bpf_recvmsg+0x7f9/0xe20 net/unix/unix_bpf.c:77
unix_stream_recvmsg+0x214/0x2c0 net/unix/af_unix.c:2832
sock_recvmsg_nosec net/socket.c:944 [inline]
sock_recvmsg net/socket.c:962 [inline]
sock_read_iter+0x3a7/0x4d0 net/socket.c:1035
call_read_iter include/linux/fs.h:2156 [inline]
io_iter_do_read fs/io_uring.c:3501 [inline]
io_read fs/io_uring.c:3558 [inline]
io_issue_sqe+0x144c/0x9590 fs/io_uring.c:6671
io_wq_submit_work+0x2d8/0x790 fs/io_uring.c:6836
io_worker_handle_work+0x808/0xdd0 fs/io-wq.c:574
io_wqe_worker+0x395/0x870 fs/io-wq.c:630
ret_from_fork+0x1f/0x30

We can safely drop the lock before doing work creation, making the two
contexts the same in that regard.

Reported-by: syzbot+b18b8be69df33a3918e9@syzkaller.appspotmail.com
Fixes: 71a85387546e ("io-wq: check for wq exit after adding new worker task_work")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/io-wq.c | 2 ++
1 file changed, 2 insertions(+)

--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -395,7 +395,9 @@ static void io_wqe_dec_running(struct io
if (atomic_dec_and_test(&acct->nr_running) && io_acct_run_queue(acct)) {
atomic_inc(&acct->nr_running);
atomic_inc(&wqe->wq->worker_refs);
+ raw_spin_unlock(&wqe->lock);
io_queue_worker_create(worker, acct, create_worker_cb);
+ raw_spin_lock(&wqe->lock);
}
}