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Date:   Fri, 11 Jun 2021 14:08:26 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+9fa589353c6e1587c6b3@...kaller.appspotmail.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in kernfs_iop_get_link

On Fri, Jun 11, 2021 at 2:01 PM syzbot
<syzbot+9fa589353c6e1587c6b3@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    614124be Linux 5.13-rc5
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12a6cdbbd00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=30f476588412c065
> dashboard link: https://syzkaller.appspot.com/bug?extid=9fa589353c6e1587c6b3
>
> 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+9fa589353c6e1587c6b3@...kaller.appspotmail.com

Concurrent reports, looks like another case of:

#syz fix: rcu: Reject RCU_LOCKDEP_WARN() false positives

> =============================
> WARNING: suspicious RCU usage
> 5.13.0-rc5-syzkaller #0 Not tainted
> -----------------------------
> kernel/sched/core.c:8304 Illegal context switch in RCU-sched read-side critical section!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 0
> no locks held by systemd-udevd/13755.
>
> stack backtrace:
> CPU: 0 PID: 13755 Comm: systemd-udevd Not tainted 5.13.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
>  ___might_sleep+0x266/0x2c0 kernel/sched/core.c:8304
>  might_alloc include/linux/sched/mm.h:201 [inline]
>  slab_pre_alloc_hook mm/slab.h:497 [inline]
>  slab_alloc_node mm/slub.c:2834 [inline]
>  slab_alloc mm/slub.c:2921 [inline]
>  kmem_cache_alloc_trace+0x277/0x2c0 mm/slub.c:2938
>  kmalloc include/linux/slab.h:556 [inline]
>  kzalloc include/linux/slab.h:686 [inline]
>  kernfs_iop_get_link fs/kernfs/symlink.c:135 [inline]
>  kernfs_iop_get_link+0x61/0x6e0 fs/kernfs/symlink.c:126
>  pick_link fs/namei.c:1741 [inline]
>  step_into+0x168e/0x1c80 fs/namei.c:1798
>  open_last_lookups fs/namei.c:3308 [inline]
>  path_openat+0x491/0x27e0 fs/namei.c:3491
>  do_filp_open+0x190/0x3d0 fs/namei.c:3521
>  do_sys_openat2+0x16d/0x420 fs/open.c:1187
>  do_sys_open fs/open.c:1203 [inline]
>  __do_sys_open fs/open.c:1211 [inline]
>  __se_sys_open fs/open.c:1207 [inline]
>  __x64_sys_open+0x119/0x1c0 fs/open.c:1207
>  do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f0ff9316840
> Code: 73 01 c3 48 8b 0d 68 77 20 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d 89 bb 20 00 00 75 10 b8 02 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 1e f6 ff ff 48 89 04 24
> RSP: 002b:00007ffeb81b55a8 EFLAGS: 00000246 ORIG_RAX: 0000000000000002
> RAX: ffffffffffffffda RBX: 0000000000000701 RCX: 00007f0ff9316840
> RDX: 0000000000000001 RSI: 0000000000080000 RDI: 00007ffeb81b55b0
> RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000012
> R10: 0000000000000064 R11: 0000000000000246 R12: 00007ffeb81b6600
> R13: 00007ffeb81b7690 R14: 0000000000000000 R15: 0000000000000000
>
>
> ---
> 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.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> --
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