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@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> =============================
> 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.