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BUG: KCSAN: data-race in snd_rawmidi_drain_output / snd_rawmidi_transmit

write to 0xffff888015b597b8 of 8 bytes by task 2011 on cpu 1:
 __snd_rawmidi_transmit_ack sound/core/rawmidi.c:1267 [inline]
 snd_rawmidi_transmit+0x2b8/0x3b0 sound/core/rawmidi.c:1326
 snd_vmidi_output_work+0xf1/0x200 sound/core/seq/seq_virmidi.c:144
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff888015b597b8 of 8 bytes by task 14576 on cpu 0:
 snd_rawmidi_drain_output+0x141/0x440 sound/core/rawmidi.c:232
 snd_rawmidi_ioctl+0x4f1/0x8c0 sound/core/rawmidi.c:905
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 14576 Comm: syz-executor.1 Tainted: G        W         5.12.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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