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BUG: KCSAN: data-race in n_tty_check_unthrottle / tty_throttle_safe

write to 0xffff888100cd4de8 of 4 bytes by task 16964 on cpu 1:
 tty_set_flow_change drivers/tty/tty.h:54 [inline]
 n_tty_check_unthrottle+0xe8/0x1d0 drivers/tty/n_tty.c:291
 n_tty_read+0x4ae/0x10c0 drivers/tty/n_tty.c:2095
 iterate_tty_read drivers/tty/tty_io.c:868 [inline]
 tty_read+0x18c/0x410 drivers/tty/tty_io.c:944
 call_read_iter include/linux/fs.h:2108 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x565/0x5d0 fs/read_write.c:496
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888100cd4de8 of 4 bytes by task 17098 on cpu 0:
 tty_throttle_safe+0x53/0xd0 drivers/tty/tty_ioctl.c:142
 n_tty_check_throttle drivers/tty/n_tty.c:264 [inline]
 n_tty_receive_buf_common+0x4e1a/0x4e70 drivers/tty/n_tty.c:1693
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1709
 tty_ldisc_receive_buf+0x5b/0xf0 drivers/tty/tty_buffer.c:471
 paste_selection+0x215/0x300 drivers/tty/vt/selection.c:408
 tioclinux+0xe5/0x450 drivers/tty/vt/vt.c:3164
 vt_ioctl+0x26e/0x2060 drivers/tty/vt/vt_ioctl.c:760
 tty_ioctl+0xa24/0x1120 drivers/tty/tty_io.c:2805
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:1069 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:1055
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:1055
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000001 -> 0x00000002

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 17098 Comm: syz-executor.4 Not tainted 5.14.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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