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BUG: KCSAN: data-race in __process_echoes / n_tty_write

write to 0xffffc90001641280 of 8 bytes by task 1687 on cpu 1:
 __process_echoes+0x63e/0x6b0 drivers/tty/n_tty.c:726
 flush_echoes drivers/tty/n_tty.c:785 [inline]
 __receive_buf drivers/tty/n_tty.c:1618 [inline]
 n_tty_receive_buf_common+0x918/0xc10 drivers/tty/n_tty.c:1715
 n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1759
 tty_ldisc_receive_buf+0x66/0xf0 drivers/tty/tty_buffer.c:387
 tty_port_default_receive_buf+0x59/0x90 drivers/tty/tty_port.c:37
 receive_buf drivers/tty/tty_buffer.c:445 [inline]
 flush_to_ldisc+0x148/0x340 drivers/tty/tty_buffer.c:495
 process_one_work kernel/workqueue.c:3257 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
 worker_thread+0x582/0x770 kernel/workqueue.c:3421
 kthread+0x489/0x510 kernel/kthread.c:463
 ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

read to 0xffffc90001641280 of 8 bytes by task 27553 on cpu 0:
 process_echoes drivers/tty/n_tty.c:762 [inline]
 n_tty_write+0x15f/0xbf0 drivers/tty/n_tty.c:2352
 iterate_tty_write drivers/tty/tty_io.c:1006 [inline]
 file_tty_write+0x378/0x690 drivers/tty/tty_io.c:1081
 tty_write+0x25/0x30 drivers/tty/tty_io.c:1102
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x52a/0x960 fs/read_write.c:686
 ksys_write+0xda/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000000000 -> 0x0000000000000007

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 27553 Comm: syz.7.8232 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
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