WARNING: lock held when returning to user space in rcu_lock_acquire

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 54ecb8f7 Linux 5.4-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11c03fcd600000
kernel config: https://syzkaller.appspot.com/x/.config?x=fb0b431ccdf08c1c
dashboard link: https://syzkaller.appspot.com/bug?extid=fef86971c84310f1c8cd
compiler: clang version 9.0.0 (/home/glider/llvm/clang
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10d52e33600000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1582402b600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+fef869...@syzkaller.appspotmail.com

================================================
WARNING: lock held when returning to user space!
5.4.0-rc1 #0 Not tainted
------------------------------------------------
syz-executor670/7923 is leaving the kernel with locks still held!
1 lock held by syz-executor670/7923:
#0: ffffffff888d3cc0 (rcu_read_lock){....}, at: rcu_lock_acquire+0x4/0x30
include/linux/rcupdate.h:207
------------[ cut here ]------------
WARNING: CPU: 0 PID: 7923 at kernel/rcu/tree_plugin.h:293
rcu_note_context_switch+0xdde/0xee0 kernel/rcu/tree_plugin.h:293
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 7923 Comm: syz-executor670 Not tainted 5.4.0-rc1 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1d8/0x2f8 lib/dump_stack.c:113
panic+0x25c/0x799 kernel/panic.c:220
__warn+0x20e/0x210 kernel/panic.c:581
report_bug+0x1b6/0x2f0 lib/bug.c:195
fixup_bug arch/x86/kernel/traps.c:179 [inline]
do_error_trap+0xd7/0x440 arch/x86/kernel/traps.c:272
do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:291
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:rcu_note_context_switch+0xdde/0xee0 kernel/rcu/tree_plugin.h:293
Code: c8 73 4b 00 e9 b8 f3 ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c 22 f3
ff ff 48 89 df e8 4b 73 4b 00 83 3b 00 0f 8e 1a f3 ff ff <0f> 0b e9 13 f3
ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c c9 f2 ff
RSP: 0000:ffff8880921bfd20 EFLAGS: 00010002
RAX: 1ffff11012bc6100 RBX: ffff888095e30978 RCX: ffffffff81608604
RDX: 0000000000000000 RSI: 0000000000000008 RDI: ffffffff88be39a0
RBP: ffff8880921bfe00 R08: dffffc0000000000 R09: fffffbfff117c735
R10: fffffbfff117c735 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff888095e30600 R14: 0000000000000000 R15: ffff8880aea35740
__schedule+0xce/0xb80 kernel/sched/core.c:4007
schedule+0x131/0x1e0 kernel/sched/core.c:4136
exit_to_usermode_loop arch/x86/entry/common.c:149 [inline]
prepare_exit_to_usermode+0x2aa/0x580 arch/x86/entry/common.c:194
retint_user+0x8/0x18
RIP: 0033:0x446fb9
Code: e8 8c 19 03 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 4b 0e fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fd680470db8 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000000 RBX: 00000000006ddc28 RCX: 0000000000446fb9
RDX: 0000000000446fb9 RSI: 0000000000000000 RDI: 0000000100000008
RBP: 00000000006ddc20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006ddc2c
R13: 00007fffb42e414f R14: 00007fd6804719c0 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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[Eric Biggers]

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#syz fix: membarrier: Fix RCU locking bug caused by faulty merge