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BUG: KCSAN: data-race in shmem_file_read_iter / shmem_write_end

write to 0xffff8881406b72c0 of 8 bytes by task 30380 on cpu 0:
 i_size_write include/linux/fs.h:932 [inline]
 shmem_write_end+0x9a/0x470 mm/shmem.c:2741
 generic_perform_write+0x267/0x410 mm/filemap.c:3941
 shmem_file_write_iter+0xc8/0xe0 mm/shmem.c:2893
 call_write_iter include/linux/fs.h:2085 [inline]
 new_sync_write fs/read_write.c:497 [inline]
 vfs_write+0x760/0x8d0 fs/read_write.c:590
 ksys_write+0xeb/0x1a0 fs/read_write.c:643
 __do_sys_write fs/read_write.c:655 [inline]
 __se_sys_write fs/read_write.c:652 [inline]
 __x64_sys_write+0x42/0x50 fs/read_write.c:652
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

read to 0xffff8881406b72c0 of 8 bytes by task 30398 on cpu 1:
 i_size_read include/linux/fs.h:910 [inline]
 shmem_file_read_iter+0x1ee/0x550 mm/shmem.c:2810
 do_iter_readv_writev+0x2b0/0x330
 vfs_readv+0x1e5/0x640 fs/read_write.c:932
 do_preadv fs/read_write.c:1049 [inline]
 __do_sys_preadv fs/read_write.c:1099 [inline]
 __se_sys_preadv fs/read_write.c:1094 [inline]
 __x64_sys_preadv+0x100/0x1c0 fs/read_write.c:1094
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

value changed: 0x0000000000a67000 -> 0x0000000000a6e000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 30398 Comm: syz-executor.2 Not tainted 6.8.0-rc3-syzkaller-00010-g6d280f4d760e #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
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