KCSAN: data-race in do_epoll_wait / ep_poll_callback

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 6116dea8 Merge tag 'kgdb-5.8-rc3' of git://git.kernel.org/..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11d0506b100000
kernel config: https://syzkaller.appspot.com/x/.config?x=d5bd81d2df2321b6
dashboard link: https://syzkaller.appspot.com/bug?extid=a865334955613363cc13
compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project.git ca2dcbd030eadbf0aa9b660efe864ff08af6e18b)
CC: [linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

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+a86533...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in do_epoll_wait / ep_poll_callback

write to 0xffff88811f80e1d0 of 8 bytes by task 8633 on cpu 0:
list_add_tail_lockless fs/eventpoll.c:1158 [inline]
ep_poll_callback+0x29c/0x510 fs/eventpoll.c:1248
__wake_up_common+0xbc/0x130 kernel/sched/wait.c:93
__wake_up_common_lock kernel/sched/wait.c:123 [inline]
__wake_up_sync_key+0x83/0xc0 kernel/sched/wait.c:190
pipe_write+0x9a6/0xbb0 fs/pipe.c:599
call_write_iter include/linux/fs.h:1907 [inline]
new_sync_write fs/read_write.c:484 [inline]
__vfs_write+0x2d8/0x340 fs/read_write.c:497
vfs_write+0x19b/0x350 fs/read_write.c:559
ksys_write+0xce/0x180 fs/read_write.c:612
__do_sys_write fs/read_write.c:624 [inline]
__se_sys_write fs/read_write.c:621 [inline]
__x64_sys_write+0x3e/0x50 fs/read_write.c:621
do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811f80e1d0 of 8 bytes by task 8632 on cpu 1:
list_empty_careful include/linux/list.h:300 [inline]
ep_events_available fs/eventpoll.c:379 [inline]
ep_poll fs/eventpoll.c:1855 [inline]
do_epoll_wait+0x2e2/0x840 fs/eventpoll.c:2333
__do_sys_epoll_pwait fs/eventpoll.c:2364 [inline]
__se_sys_epoll_pwait fs/eventpoll.c:2350 [inline]
__x64_sys_epoll_pwait+0x92/0x140 fs/eventpoll.c:2350
do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8632 Comm: syz-fuzzer Not tainted 5.8.0-rc2-syzkaller #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.