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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 9/13/23 14:07, Marco Elver wrote: > On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov wrote: >> >> 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. > > It's a simple bit flag set & read, I'd go for READ_ONCE() (and > WRITE_ONCE() - but up to you, these bitflag sets & reads have been ok > with just the READ_ONCE(), and KCSAN currently doesn't care if there's > a WRITE_ONCE() or not). > >> value changed: 0x0000000d -> 0x0000000b > > This is interesting though - it says that it observed 2 bits being > flipped. We don't see where IO_WORKER_F_FREE was unset though. __io_worker_busy() clears it, should be it. I assume syz just missed another false data race with this one. After init only the worker thread should be changing the flags AFAIR -- Pavel Begunkov