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BUG: KCSAN: data-race in snd_pcm_drain / snd_pcm_start

write to 0xffff888121f82000 of 4 bytes by task 11293 on cpu 1:
 snd_pcm_post_start sound/core/pcm_native.c:1368 [inline]
 snd_pcm_action_single sound/core/pcm_native.c:1209 [inline]
 snd_pcm_action sound/core/pcm_native.c:1290 [inline]
 snd_pcm_start+0x31f/0x4b0 sound/core/pcm_native.c:1391
 __snd_pcm_lib_xfer+0xf15/0x10e0 sound/core/pcm_lib.c:2247
 snd_pcm_oss_write3+0x192/0x220 sound/core/oss/pcm_oss.c:1221
 snd_pcm_oss_write2+0x13a/0x250 sound/core/oss/pcm_oss.c:1361
 snd_pcm_oss_write1 sound/core/oss/pcm_oss.c:1419 [inline]
 snd_pcm_oss_write+0x313/0x890 sound/core/oss/pcm_oss.c:2765
 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 0xffff888121f82000 of 4 bytes by task 11297 on cpu 0:
 snd_pcm_drain+0x82/0xa20 sound/core/pcm_native.c:2014
 snd_pcm_kernel_ioctl+0x1ae/0xc20 sound/core/pcm_native.c:3333
 snd_pcm_oss_sync+0xb2/0x680 sound/core/oss/pcm_oss.c:1697
 snd_pcm_oss_ioctl+0x613/0x2970 sound/core/oss/pcm_oss.c:2590
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 11297 Comm: syz-executor.2 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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