[syzbot] possible deadlock in io_sq_thread

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a2b28235335f Merge branch 'dmi-for-linus' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=175ec47d300000
kernel config: https://syzkaller.appspot.com/x/.config?x=f5102874b5591af7
dashboard link: https://syzkaller.appspot.com/bug?extid=c9e7f2674197989e8afb
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c9e7f2...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
5.14.0-syzkaller #0 Not tainted
------------------------------------------------------
iou-sqp-17493/17496 is trying to acquire lock:
ffff888084a320a8 (&ctx->uring_lock){+.+.}-{3:3}, at: __io_sq_thread fs/io_uring.c:7291 [inline]
ffff888084a320a8 (&ctx->uring_lock){+.+.}-{3:3}, at: io_sq_thread+0x65a/0x1370 fs/io_uring.c:7368

but task is already holding lock:
ffff888012986870 (&sqd->lock){+.+.}-{3:3}, at: io_sq_thread+0x20f/0x1370 fs/io_uring.c:7356

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&sqd->lock){+.+.}-{3:3}:
__mutex_lock_common kernel/locking/mutex.c:596 [inline]
__mutex_lock+0x131/0x12f0 kernel/locking/mutex.c:729
io_register_iowq_max_workers fs/io_uring.c:10551 [inline]
__io_uring_register fs/io_uring.c:10757 [inline]
__do_sys_io_uring_register+0x10aa/0x2e70 fs/io_uring.c:10792
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #0 (&ctx->uring_lock){+.+.}-{3:3}:
check_prev_add kernel/locking/lockdep.c:3051 [inline]
check_prevs_add kernel/locking/lockdep.c:3174 [inline]
validate_chain kernel/locking/lockdep.c:3789 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5015
lock_acquire kernel/locking/lockdep.c:5625 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5590
__mutex_lock_common kernel/locking/mutex.c:596 [inline]
__mutex_lock+0x131/0x12f0 kernel/locking/mutex.c:729
__io_sq_thread fs/io_uring.c:7291 [inline]
io_sq_thread+0x65a/0x1370 fs/io_uring.c:7368
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sqd->lock);
lock(&ctx->uring_lock);
lock(&sqd->lock);
lock(&ctx->uring_lock);

*** DEADLOCK ***

1 lock held by iou-sqp-17493/17496:
#0: ffff888012986870 (&sqd->lock){+.+.}-{3:3}, at: io_sq_thread+0x20f/0x1370 fs/io_uring.c:7356

stack backtrace:
CPU: 0 PID: 17496 Comm: iou-sqp-17493 Not tainted 5.14.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:105
check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2131
check_prev_add kernel/locking/lockdep.c:3051 [inline]
check_prevs_add kernel/locking/lockdep.c:3174 [inline]
validate_chain kernel/locking/lockdep.c:3789 [inline]
__lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5015
lock_acquire kernel/locking/lockdep.c:5625 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5590
__mutex_lock_common kernel/locking/mutex.c:596 [inline]
__mutex_lock+0x131/0x12f0 kernel/locking/mutex.c:729
__io_sq_thread fs/io_uring.c:7291 [inline]
io_sq_thread+0x65a/0x1370 fs/io_uring.c:7368
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295


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[Jens Axboe]

On 9/8/21 6:10 PM, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: a2b28235335f Merge branch 'dmi-for-linus' of git://git.ker..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=175ec47d300000
> kernel config: https://syzkaller.appspot.com/x/.config?x=f5102874b5591af7
> dashboard link: https://syzkaller.appspot.com/bug?extid=c9e7f2674197989e8afb
> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1

#syz dup: [syzbot] possible deadlock in io_uring_register

--
Jens Axboe