On Thu, 14 Sept 2023 at 15:11, Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: > > On 9/13/23 14:07, Marco Elver wrote: > > On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov <asml.silence@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx > >>> > >>> ================================================================== > >>> 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 are benign or not. If benign, you can choose to annotate with 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
