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Message-ID: <4e400095-7205-883b-c8fd-4aa95a1b6423@gmail.com>
Date:   Wed, 13 Sep 2023 13:12:22 +0100
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Subject: Re: [syzbot] [io-uring?] KCSAN: data-race in
 io_wq_activate_free_worker / io_wq_worker_running
To:     syzbot <syzbot+a36975231499dc24df44@syzkaller.appspotmail.com>,
        axboe@kernel.dk, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
References: <000000000000fc6ba706053be013@google.com>
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From:   Pavel Begunkov <asml.silence@gmail.com>
In-Reply-To: <000000000000fc6ba706053be013@google.com>
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On 9/13/23 12:29, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    f97e18a3f2fb Merge tag 'gpio-updates-for-v6.6' of git://gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12864667a80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=fe440f256d065d3b
> dashboard link: https://syzkaller.appspot.com/bug?extid=a36975231499dc24df44
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/b1781aaff038/disk-f97e18a3.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/5b915468fd6d/vmlinux-f97e18a3.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/abc8ece931f3/bzImage-f97e18a3.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+a36975231499dc24df44@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
> 
> write to 0xffff888127f736c4 of 4 bytes by task 4731 on cpu 1:
>   io_wq_worker_running+0x64/0xa0 io_uring/io-wq.c:668
>   schedule_timeout+0xcc/0x230 kernel/time/timer.c:2167
>   io_wq_worker+0x4b2/0x840 io_uring/io-wq.c:633
>   ret_from_fork+0x2e/0x40 arch/x86/kernel/process.c:145
>   ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304
> 
> read to 0xffff888127f736c4 of 4 bytes by task 4719 on cpu 0:
>   io_wq_get_acct io_uring/io-wq.c:168 [inline]
>   io_wq_activate_free_worker+0xfa/0x280 io_uring/io-wq.c:267
>   io_wq_enqueue+0x262/0x450 io_uring/io-wq.c:914

1) the worst case scenario we'll choose a wrong type of
worker, which is inconsequential.

2) we're changing the IO_WORKER_F_RUNNING bit, but checking
for IO_WORKER_F_BOUND. The latter one is set at the very
beginning, it would require compiler to be super inventive
to actually hit the problem.

I don't believe it's a problem, but it'll nice to attribute
it properly, READ_ONCE?, or split IO_WORKER_F_BOUND out into
a separate field.


>   io_queue_iowq+0x1d1/0x310 io_uring/io_uring.c:514
>   io_queue_sqe_fallback+0x82/0xe0 io_uring/io_uring.c:2084
>   io_submit_sqe io_uring/io_uring.c:2305 [inline]
>   io_submit_sqes+0xbd3/0xfb0 io_uring/io_uring.c:2420
>   __do_sys_io_uring_enter io_uring/io_uring.c:3628 [inline]
>   __se_sys_io_uring_enter+0x1f8/0x1c10 io_uring/io_uring.c:3562
>   __x64_sys_io_uring_enter+0x78/0x90 io_uring/io_uring.c:3562
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> value changed: 0x0000000d -> 0x0000000b
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 4719 Comm: syz-executor.1 Not tainted 6.5.0-syzkaller-01810-gf97e18a3f2fb #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
> ==================================================================
> 
> 
> ---
> 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 bug is already fixed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want to overwrite bug's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the bug is a duplicate of another bug, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup

-- 
Pavel Begunkov