KCSAN: data-race in snd_rawmidi_poll / snd_rawmidi_proceed

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 00e4db51 Merge tag 'perf-tools-2020-08-10' of git://git.ke..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12a52126900000
kernel config: https://syzkaller.appspot.com/x/.config?x=997a92ee4b5588ef
dashboard link: https://syzkaller.appspot.com/bug?extid=3d367d1df1d2b67f5c19
compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project.git ca2dcbd030eadbf0aa9b660efe864ff08af6e18b)
CC: [alsa-...@alsa-project.org ar...@arndb.de baoli...@linaro.org linux-...@vger.kernel.org pe...@perex.cz ti...@suse.com]

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+3d367d...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in snd_rawmidi_poll / snd_rawmidi_proceed

write to 0xffff8880b532fdb8 of 8 bytes by task 3254 on cpu 0:
__snd_rawmidi_transmit_ack sound/core/rawmidi.c:1255 [inline]
snd_rawmidi_proceed+0x126/0x260 sound/core/rawmidi.c:1336
snd_vmidi_output_work+0x8e/0x200 sound/core/seq/seq_virmidi.c:139
process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
kthread+0x20d/0x230 kernel/kthread.c:292
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff8880b532fdb8 of 8 bytes by task 15016 on cpu 1:
snd_rawmidi_ready sound/core/rawmidi.c:102 [inline]
snd_rawmidi_poll+0x1ce/0x210 sound/core/rawmidi.c:1506
vfs_poll include/linux/poll.h:90 [inline]
do_select+0x8f6/0xf70 fs/select.c:534
core_sys_select+0x3e3/0x610 fs/select.c:677
do_pselect fs/select.c:759 [inline]
__do_sys_pselect6 fs/select.c:800 [inline]
__se_sys_pselect6+0x248/0x290 fs/select.c:791
__x64_sys_pselect6+0x74/0x80 fs/select.c:791
do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 15016 Comm: syz-executor.3 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================


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