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BUG: KCSAN: data-race in snd_seq_oss_readq_poll / snd_seq_oss_readq_put_event

write to 0xffff888121f6ae08 of 4 bytes by task 3447 on cpu 1:
 snd_seq_oss_readq_put_event+0x103/0x150 sound/core/seq/oss/seq_oss_readq.c:153
 send_synth_event sound/core/seq/oss/seq_oss_midi.c:586 [inline]
 snd_seq_oss_midi_input+0x4ea/0x500 sound/core/seq/oss/seq_oss_midi.c:522
 snd_seq_oss_event_input+0xbc/0x100 sound/core/seq/oss/seq_oss_event.c:429
 snd_seq_deliver_single_event+0x31b/0x4f0 sound/core/seq/seq_clientmgr.c:638
 deliver_to_subscribers sound/core/seq/seq_clientmgr.c:693 [inline]
 snd_seq_deliver_event+0x392/0x4a0 sound/core/seq/seq_clientmgr.c:828
 snd_seq_kernel_client_dispatch+0x16f/0x190 sound/core/seq/seq_clientmgr.c:2323
 dummy_input+0xe3/0x110 sound/core/seq/seq_dummy.c:90
 snd_seq_deliver_single_event+0x31b/0x4f0 sound/core/seq/seq_clientmgr.c:638
 snd_seq_deliver_event+0x192/0x4a0 sound/core/seq/seq_clientmgr.c:839
 snd_seq_kernel_client_dispatch+0x16f/0x190 sound/core/seq/seq_clientmgr.c:2323
 snd_seq_oss_dispatch sound/core/seq/oss/seq_oss_device.h:138 [inline]
 snd_seq_oss_midi_reset+0x24b/0x2d0 sound/core/seq/oss/seq_oss_midi.c:474
 snd_seq_oss_synth_reset+0x281/0x4a0 sound/core/seq/oss/seq_oss_synth.c:404
 snd_seq_oss_reset+0x3b/0x160 sound/core/seq/oss/seq_oss_init.c:435
 snd_seq_oss_ioctl+0x48b/0xa50 sound/core/seq/oss/seq_oss_ioctl.c:93
 odev_ioctl+0x47/0x70 sound/core/seq/oss/seq_oss.c:178
 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+0x44/0xa9

read to 0xffff888121f6ae08 of 4 bytes by task 3445 on cpu 0:
 snd_seq_oss_readq_poll+0x5d/0x70 sound/core/seq/oss/seq_oss_readq.c:209
 snd_seq_oss_poll+0x73/0x110 sound/core/seq/oss/seq_oss_rw.c:190
 odev_poll+0x3b/0x50 sound/core/seq/oss/seq_oss.c:200
 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: 0 PID: 3445 Comm: syz-executor.0 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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