WARNING: suspicious RCU usage in count

WARNING: suspicious RCU usage in count

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    7f376f19 Merge tag 'mtd/fixes-for-5.10-rc8' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1217bb7f500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ee8a1012a5314210
dashboard link: https://syzkaller.appspot.com/bug?extid=51ce7a5794c3b12a70d1
compiler:       gcc (GCC) 10.1.0-syz 20200507
userspace arch: i386

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+51ce7a5794c3b12a70d1@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
5.10.0-rc7-syzkaller #0 Not tainted
-----------------------------
kernel/sched/core.c:7270 Illegal context switch in RCU-bh read-side critical section!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 0
no locks held by udevd/9038.

stack backtrace:
CPU: 3 PID: 9038 Comm: udevd Not tainted 5.10.0-rc7-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x107/0x163 lib/dump_stack.c:118
 ___might_sleep+0x220/0x2b0 kernel/sched/core.c:7270
 count.constprop.0+0x164/0x270 fs/exec.c:449
 do_execveat_common+0x2fd/0x7c0 fs/exec.c:1893
 do_execve fs/exec.c:1983 [inline]
 __do_sys_execve fs/exec.c:2059 [inline]
 __se_sys_execve fs/exec.c:2054 [inline]
 __x64_sys_execve+0x8f/0xc0 fs/exec.c:2054
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fbc6f64c207
Code: 77 19 f4 48 89 d7 44 89 c0 0f 05 48 3d 00 f0 ff ff 76 e0 f7 d8 64 41 89 01 eb d8 f7 d8 64 41 89 01 eb df b8 3b 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 02 f3 c3 48 8b 15 00 8c 2d 00 f7 d8 64 89 02
RSP: 002b:00007ffcfe4966d8 EFLAGS: 00000206
 ORIG_RAX: 000000000000003b
RAX: ffffffffffffffda RBX: 00000000ffffffff RCX: 00007fbc6f64c207
RDX: 0000000000f1dfd0 RSI: 00007ffcfe4967d0 RDI: 00007ffcfe4977e0
RBP: 0000000000625500 R08: 00000000000022fb R09: 00000000000022fb
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000f1dfd0
R13: 0000000000000007 R14: 0000000000f0e250 R15: 0000000000000005


---
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Re: WARNING: suspicious RCU usage in count

[Matthew Wilcox]

On Wed, Dec 16, 2020 at 11:34:10AM -0800, syzbot wrote:
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+51ce7a5794c3b12a70d1@syzkaller.appspotmail.com
> 
> =============================
> WARNING: suspicious RCU usage
> 5.10.0-rc7-syzkaller #0 Not tainted
> -----------------------------
> kernel/sched/core.c:7270 Illegal context switch in RCU-bh read-side critical section!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 2, debug_locks = 0
> no locks held by udevd/9038.
> 
> stack backtrace:
> CPU: 3 PID: 9038 Comm: udevd Not tainted 5.10.0-rc7-syzkaller #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x107/0x163 lib/dump_stack.c:118
>  ___might_sleep+0x220/0x2b0 kernel/sched/core.c:7270
>  count.constprop.0+0x164/0x270 fs/exec.c:449
>  do_execveat_common+0x2fd/0x7c0 fs/exec.c:1893
>  do_execve fs/exec.c:1983 [inline]
>  __do_sys_execve fs/exec.c:2059 [inline]
>  __se_sys_execve fs/exec.c:2054 [inline]
>  __x64_sys_execve+0x8f/0xc0 fs/exec.c:2054
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46

This must be the victim of something else.  There's no way this call
trace took the RCU read lock.

Re: WARNING: suspicious RCU usage in count

[Dmitry Vyukov]

On Wed, Dec 16, 2020 at 9:55 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Dec 16, 2020 at 11:34:10AM -0800, syzbot wrote:
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+51ce7a5794c3b12a70d1@syzkaller.appspotmail.com
> >
> > =============================
> > WARNING: suspicious RCU usage
> > 5.10.0-rc7-syzkaller #0 Not tainted
> > -----------------------------
> > kernel/sched/core.c:7270 Illegal context switch in RCU-bh read-side critical section!
> >
> > other info that might help us debug this:
> >
> >
> > rcu_scheduler_active = 2, debug_locks = 0
> > no locks held by udevd/9038.
> >
> > stack backtrace:
> > CPU: 3 PID: 9038 Comm: udevd Not tainted 5.10.0-rc7-syzkaller #0
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x107/0x163 lib/dump_stack.c:118
> >  ___might_sleep+0x220/0x2b0 kernel/sched/core.c:7270
> >  count.constprop.0+0x164/0x270 fs/exec.c:449
> >  do_execveat_common+0x2fd/0x7c0 fs/exec.c:1893
> >  do_execve fs/exec.c:1983 [inline]
> >  __do_sys_execve fs/exec.c:2059 [inline]
> >  __se_sys_execve fs/exec.c:2054 [inline]
> >  __x64_sys_execve+0x8f/0xc0 fs/exec.c:2054
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>
> This must be the victim of something else.  There's no way this call
> trace took the RCU read lock.

+lockdep maintainers for lockdep false positive then and +Paul for rcu

There is another recent claim of a false "suspicious RCU usage":
https://lore.kernel.org/lkml/CAKMK7uEiS5SrBYv-2w2wWL=9G4ByoHvtiWVsPqekswZzOGmzjg@mail.gmail.com/

Re: WARNING: suspicious RCU usage in count

[Paul E. McKenney]

On Thu, Dec 17, 2020 at 09:26:44AM +0100, Dmitry Vyukov wrote:
> On Wed, Dec 16, 2020 at 9:55 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Dec 16, 2020 at 11:34:10AM -0800, syzbot wrote:
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+51ce7a5794c3b12a70d1@syzkaller.appspotmail.com
> > >
> > > =============================
> > > WARNING: suspicious RCU usage
> > > 5.10.0-rc7-syzkaller #0 Not tainted
> > > -----------------------------
> > > kernel/sched/core.c:7270 Illegal context switch in RCU-bh read-side critical section!
> > >
> > > other info that might help us debug this:
> > >
> > >
> > > rcu_scheduler_active = 2, debug_locks = 0
> > > no locks held by udevd/9038.
> > >
> > > stack backtrace:
> > > CPU: 3 PID: 9038 Comm: udevd Not tainted 5.10.0-rc7-syzkaller #0
> > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> > > Call Trace:
> > >  __dump_stack lib/dump_stack.c:77 [inline]
> > >  dump_stack+0x107/0x163 lib/dump_stack.c:118
> > >  ___might_sleep+0x220/0x2b0 kernel/sched/core.c:7270
> > >  count.constprop.0+0x164/0x270 fs/exec.c:449
> > >  do_execveat_common+0x2fd/0x7c0 fs/exec.c:1893
> > >  do_execve fs/exec.c:1983 [inline]
> > >  __do_sys_execve fs/exec.c:2059 [inline]
> > >  __se_sys_execve fs/exec.c:2054 [inline]
> > >  __x64_sys_execve+0x8f/0xc0 fs/exec.c:2054
> > >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >
> > This must be the victim of something else.  There's no way this call
> > trace took the RCU read lock.
> 
> +lockdep maintainers for lockdep false positive then and +Paul for rcu

Note that this was "RCU-bh" rather than "RCU", so it might be something
like local_bh_disable() rather than rcu_read_lock() that might_sleep()
is complaining about.

> There is another recent claim of a false "suspicious RCU usage":
> https://lore.kernel.org/lkml/CAKMK7uEiS5SrBYv-2w2wWL=9G4ByoHvtiWVsPqekswZzOGmzjg@mail.gmail.com/

That one does look familiar.  ;-)

							Thanx, Paul