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BUG: KCSAN: data-race in snd_rawmidi_kernel_write1 / snd_rawmidi_poll

write to 0xffff8880b68bc1b8 of 8 bytes by task 20378 on cpu 1:
 snd_rawmidi_kernel_write1+0x1e1/0x430 sound/core/rawmidi.c:1378
 snd_rawmidi_write+0x31b/0x6b0 sound/core/rawmidi.c:1452
 vfs_write+0x1df/0x6f0 fs/read_write.c:576
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b68bc1b8 of 8 bytes by task 20382 on cpu 0:
 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_pollfd fs/select.c:869 [inline]
 do_poll fs/select.c:917 [inline]
 do_sys_poll+0x606/0xbc0 fs/select.c:1011
 __do_sys_ppoll fs/select.c:1111 [inline]
 __se_sys_ppoll+0x169/0x1b0 fs/select.c:1091
 __x64_sys_ppoll+0x63/0x70 fs/select.c:1091
 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: 0 PID: 20382 Comm: syz-executor.5 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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