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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 9/14/23 14:25, Marco Elver wrote: > On Thu, 14 Sept 2023 at 15:11, Pavel Begunkov wrote: >> >> 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 > > The data races reported are very real, i.e. it only reports if it > actually observes _real_ concurrency. I guess the question is if these That's what I'm saying, I assume that syz is not completely analytical and triggering a race is subject to execution randomness, and races with IO_WORKER_F_FREE are harder to hit for syzkaller. > are benign or not. If benign, you can choose to annotate with Yes, it is, just like the one in the report > READ/WRITE_ONCE [1], data_race, or leave as is (ignoring this report > should not make it re-report any time soon). > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt -- Pavel Begunkov