* [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
@ 2023-09-13 11:29 syzbot
2023-09-13 12:12 ` Pavel Begunkov
0 siblings, 1 reply; 6+ messages in thread
From: syzbot @ 2023-09-13 11:29 UTC (permalink / raw)
To: asml.silence, axboe, io-uring, linux-kernel, syzkaller-bugs
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: [email protected]
==================================================================
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
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 [email protected].
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
2023-09-13 11:29 [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running syzbot
@ 2023-09-13 12:12 ` Pavel Begunkov
2023-09-13 13:07 ` Marco Elver
0 siblings, 1 reply; 6+ messages in thread
From: Pavel Begunkov @ 2023-09-13 12:12 UTC (permalink / raw)
To: syzbot, axboe, io-uring, linux-kernel, syzkaller-bugs
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: [email protected]
>
> ==================================================================
> 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 [email protected].
>
> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
2023-09-13 12:12 ` Pavel Begunkov
@ 2023-09-13 13:07 ` Marco Elver
2023-09-14 13:09 ` Pavel Begunkov
0 siblings, 1 reply; 6+ messages in thread
From: Marco Elver @ 2023-09-13 13:07 UTC (permalink / raw)
To: Pavel Begunkov; +Cc: syzbot, axboe, io-uring, linux-kernel, syzkaller-bugs
On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov <[email protected]> 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: [email protected]
> >
> > ==================================================================
> > 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.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
2023-09-13 13:07 ` Marco Elver
@ 2023-09-14 13:09 ` Pavel Begunkov
2023-09-14 13:25 ` Marco Elver
0 siblings, 1 reply; 6+ messages in thread
From: Pavel Begunkov @ 2023-09-14 13:09 UTC (permalink / raw)
To: Marco Elver; +Cc: syzbot, axboe, io-uring, linux-kernel, syzkaller-bugs
On 9/13/23 14:07, Marco Elver wrote:
> On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov <[email protected]> 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: [email protected]
>>>
>>> ==================================================================
>>> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
2023-09-14 13:09 ` Pavel Begunkov
@ 2023-09-14 13:25 ` Marco Elver
2023-09-14 13:39 ` Pavel Begunkov
0 siblings, 1 reply; 6+ messages in thread
From: Marco Elver @ 2023-09-14 13:25 UTC (permalink / raw)
To: Pavel Begunkov; +Cc: syzbot, axboe, io-uring, linux-kernel, syzkaller-bugs
On Thu, 14 Sept 2023 at 15:11, Pavel Begunkov <[email protected]> wrote:
>
> On 9/13/23 14:07, Marco Elver wrote:
> > On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov <[email protected]> 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: [email protected]
> >>>
> >>> ==================================================================
> >>> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running
2023-09-14 13:25 ` Marco Elver
@ 2023-09-14 13:39 ` Pavel Begunkov
0 siblings, 0 replies; 6+ messages in thread
From: Pavel Begunkov @ 2023-09-14 13:39 UTC (permalink / raw)
To: Marco Elver; +Cc: syzbot, axboe, io-uring, linux-kernel, syzkaller-bugs
On 9/14/23 14:25, Marco Elver wrote:
> On Thu, 14 Sept 2023 at 15:11, Pavel Begunkov <[email protected]> wrote:
>>
>> On 9/13/23 14:07, Marco Elver wrote:
>>> On Wed, 13 Sept 2023 at 14:13, Pavel Begunkov <[email protected]> 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: [email protected]
>>>>>
>>>>> ==================================================================
>>>>> 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
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-09-14 13:42 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2023-09-13 11:29 [syzbot] [io-uring?] KCSAN: data-race in io_wq_activate_free_worker / io_wq_worker_running syzbot
2023-09-13 12:12 ` Pavel Begunkov
2023-09-13 13:07 ` Marco Elver
2023-09-14 13:09 ` Pavel Begunkov
2023-09-14 13:25 ` Marco Elver
2023-09-14 13:39 ` Pavel Begunkov
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