linux-kernel - Re: [syzbot] [rcu?] WARNING in rcu_note_context

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Message-ID: <2897a6bd-6c7d-4b9b-8891-27051df45f8e@paulmck-laptop>
Date: Sun, 21 Jul 2024 09:37:40 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: syzbot <syzbot+784d0a1246a539975f05@...kaller.appspotmail.com>
Cc: boqun.feng@...il.com, frederic@...nel.org, jiangshanlai@...il.com,
	joel@...lfernandes.org, josh@...htriplett.org,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	neeraj.upadhyay@...nel.org, qiang.zhang1211@...il.com,
	rcu@...r.kernel.org, rostedt@...dmis.org,
	syzkaller-bugs@...glegroups.com, urezki@...il.com
Subject: Re: [syzbot] [rcu?] WARNING in rcu_note_context_switch (2)

On Sun, Jul 21, 2024 at 05:53:21AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    51835949dda3 Merge tag 'net-next-6.11' of git://git.kernel..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=150e825e980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=28bac69fa31fbb3a
> dashboard link: https://syzkaller.appspot.com/bug?extid=784d0a1246a539975f05
> compiler:       aarch64-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> userspace arch: arm64
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15d4bf4e980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17a3c349980000
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/384ffdcca292/non_bootable_disk-51835949.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/7a3a01db5542/vmlinux-51835949.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/14d329019155/Image-51835949.gz.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+784d0a1246a539975f05@...kaller.appspotmail.com
> 
> ------------[ cut here ]------------
> Voluntary context switch within RCU read-side critical section!
> WARNING: CPU: 0 PID: 3460 at kernel/rcu/tree_plugin.h:330 rcu_note_context_switch+0x354/0x49c kernel/rcu/tree_plugin.h:330

Taking a voluntary context switch in an RCU read-side critical section
voids your RCU warranty.

> Modules linked in:
> CPU: 0 PID: 3460 Comm: syz-executor248 Not tainted 6.10.0-syzkaller-04472-g51835949dda3 #0
> Hardware name: linux,dummy-virt (DT)
> pstate: 614000c9 (nZCv daIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
> pc : rcu_note_context_switch+0x354/0x49c kernel/rcu/tree_plugin.h:330
> lr : rcu_note_context_switch+0x354/0x49c kernel/rcu/tree_plugin.h:330
> sp : ffff800089523d30
> x29: ffff800089523d30 x28: f6f0000005d4a480 x27: 0000000000000000
> x26: 0000000000000000 x25: f6f0000005d4a480 x24: ffff800082643318
> x23: 0000000000000000 x22: f6f0000005d4a480 x21: fff000007f8d6240
> x20: ffff80008261e040 x19: fff000007f8d7040 x18: fffffffffffcb658
> x17: fff07ffffd2b9000 x16: ffff800080000000 x15: 0000000000000048
> x14: fffffffffffcb6a0 x13: ffff80008266b0a8 x12: 000000000000088b
> x11: 00000000000002d9 x10: ffff80008271f500 x9 : ffff80008266b0a8
> x8 : 00000000ffffdfff x7 : ffff80008271b0a8 x6 : 00000000000002d9
> x5 : fff000007f8cbf48 x4 : 40000000ffffe2d9 x3 : fff07ffffd2b9000
> x2 : 0000000000000000 x1 : 0000000000000000 x0 : f6f0000005d4a480
> Call trace:
>  rcu_note_context_switch+0x354/0x49c kernel/rcu/tree_plugin.h:330
>  __schedule+0xb0/0x850 kernel/sched/core.c:6417
>  __schedule_loop kernel/sched/core.c:6606 [inline]
>  schedule+0x34/0x104 kernel/sched/core.c:6621
>  do_notify_resume+0xe4/0x164 arch/arm64/kernel/entry-common.c:136
>  exit_to_user_mode_prepare arch/arm64/kernel/entry-common.c:169 [inline]
>  exit_to_user_mode arch/arm64/kernel/entry-common.c:178 [inline]
>  el0_interrupt+0xc4/0xc8 arch/arm64/kernel/entry-common.c:797
>  __el0_irq_handler_common+0x18/0x24 arch/arm64/kernel/entry-common.c:802
>  el0t_64_irq_handler+0x10/0x1c arch/arm64/kernel/entry-common.c:807
>  el0t_64_irq+0x19c/0x1a0 arch/arm64/kernel/entry.S:599
> ---[ end trace 0000000000000000 ]---

If we are exiting to user mode, my first guess would be that someone
did rcu_read_lock() but forgot the matching rcu_read_unlock().

Can this be reproduced using lockdep?  That would pinpoint the unmatched
rcu_read_lock().

							Thanx, Paul

> ---
> 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 syzkaller@...glegroups.com.
> 
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