syzbot

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BUG: KCSAN: data-race in __process_echoes / n_tty_write

write to 0xffffc90003f05280 of 8 bytes by task 13126 on cpu 1:
 __process_echoes+0x63e/0x6b0 drivers/tty/n_tty.c:730
 flush_echoes drivers/tty/n_tty.c:791 [inline]
 __receive_buf drivers/tty/n_tty.c:1626 [inline]
 n_tty_receive_buf_common+0x8e4/0xbe0 drivers/tty/n_tty.c:1723
 n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1763
 tiocsti+0x170/0x1d0 drivers/tty/tty_io.c:2290
 tty_ioctl+0x4a4/0xb80 drivers/tty/tty_io.c:2706
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffc90003f05280 of 8 bytes by task 13124 on cpu 0:
 process_echoes drivers/tty/n_tty.c:768 [inline]
 n_tty_write+0x161/0xb40 drivers/tty/n_tty.c:2363
 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+0x2802/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000000000 -> 0x0000000000000003

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 13124 Comm: syz.4.3281 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
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