KCSAN: data-race in do_syslog / log_store

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 05f22368 x86, kcsan: Enable KCSAN for x86
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=10fe752f600000
kernel config: https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=8f5c0f7fd3c70ea71b5c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [bri...@lists.linux-foundation.org da...@davemloft.net
linux-...@vger.kernel.org net...@vger.kernel.org
nik...@cumulusnetworks.com ro...@cumulusnetworks.com el...@google.com]

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

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+8f5c0f...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in do_syslog / log_store

write to 0xffffffff86040010 of 8 bytes by task 9097 on cpu 1:
log_store+0x2a9/0x430 kernel/printk/printk.c:660
log_output kernel/printk/printk.c:1899 [inline]
vprintk_store+0x297/0x430 kernel/printk/printk.c:1950
vprintk_emit+0x146/0x4e0 kernel/printk/printk.c:1978
vprintk_default+0x31/0x50 kernel/printk/printk.c:2023
vprintk_func+0x85/0x140 kernel/printk/printk_safe.c:386
printk+0x69/0x91 kernel/printk/printk.c:2056
br_set_state net/bridge/br_stp.c:45 [inline]
br_set_state.cold+0x7c/0xf5 net/bridge/br_stp.c:29
br_make_forwarding+0xb9/0x190 net/bridge/br_stp.c:417
br_port_state_selection+0x2cb/0x330 net/bridge/br_stp.c:449
br_stp_enable_port+0x30/0x50 net/bridge/br_stp_if.c:94
br_port_carrier_check+0x14f/0x250 net/bridge/br_if.c:80
br_device_event+0x241/0x570 net/bridge/br.c:86
notifier_call_chain+0xd7/0x160 kernel/notifier.c:95
__raw_notifier_call_chain kernel/notifier.c:396 [inline]
raw_notifier_call_chain+0x37/0x50 kernel/notifier.c:403

read to 0xffffffff86040010 of 8 bytes by task 7112 on cpu 0:
do_syslog kernel/printk/printk.c:1525 [inline]
do_syslog+0x12dc/0x13f0 kernel/printk/printk.c:1503
kmsg_read+0x80/0xa0 fs/proc/kmsg.c:40
proc_reg_read+0xe9/0x140 fs/proc/inode.c:223
__vfs_read+0x67/0xc0 fs/read_write.c:425
vfs_read fs/read_write.c:461 [inline]
vfs_read+0x143/0x2c0 fs/read_write.c:446
ksys_read+0xd5/0x1b0 fs/read_write.c:587
__do_sys_read fs/read_write.c:597 [inline]
__se_sys_read fs/read_write.c:595 [inline]
__x64_sys_read+0x4c/0x60 fs/read_write.c:595
do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7112 Comm: rsyslogd Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.