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BUG: KCSAN: data-race in pipe_poll / pipe_write

write to 0xffff88817a948978 of 4 bytes by task 10670 on cpu 1:
 pipe_poll+0x35/0x260 fs/pipe.c:654
 vfs_poll include/linux/poll.h:90 [inline]
 p9_fd_poll net/9p/trans_fd.c:235 [inline]
 p9_conn_create+0x218/0x300 net/9p/trans_fd.c:599
 p9_fd_create+0x1e3/0x1f0 net/9p/trans_fd.c:1078
 p9_client_create+0x59a/0x720 net/9p/client.c:1043
 v9fs_session_init+0x100/0xd50 fs/9p/v9fs.c:407
 v9fs_mount+0x57/0x4c0 fs/9p/vfs_super.c:126
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:610
 vfs_get_tree+0x4a/0x1a0 fs/super.c:1498
 do_new_mount fs/namespace.c:2988 [inline]
 path_mount+0x11ec/0x1d20 fs/namespace.c:3318
 do_mount fs/namespace.c:3331 [inline]
 __do_sys_mount fs/namespace.c:3539 [inline]
 __se_sys_mount+0x24b/0x2f0 fs/namespace.c:3516
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3516
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88817a948978 of 4 bytes by task 10666 on cpu 0:
 pipe_write+0x94b/0xd00 fs/pipe.c:591
 call_write_iter include/linux/fs.h:2163 [inline]
 new_sync_write fs/read_write.c:507 [inline]
 vfs_write+0x7bc/0x900 fs/read_write.c:594
 ksys_write+0xd9/0x190 fs/read_write.c:647
 __do_sys_write fs/read_write.c:659 [inline]
 __se_sys_write fs/read_write.c:656 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:656
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000000 -> 0x00000001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10666 Comm: syz-executor.3 Not tainted 5.15.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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