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BUG: KCSAN: data-race in shmem_file_read_iter / shmem_write_end

write to 0xffff88810e814410 of 8 bytes by task 3829 on cpu 1:
 i_size_write include/linux/fs.h:870 [inline]
 shmem_write_end+0x62/0x630 mm/shmem.c:2539
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3419
 __generic_file_write_iter+0x15c/0x330 mm/filemap.c:3537
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3569
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6d4/0x7c0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810e814410 of 8 bytes by task 3853 on cpu 0:
 i_size_read include/linux/fs.h:848 [inline]
 shmem_file_read_iter+0x232/0x760 mm/shmem.c:2622
 call_read_iter include/linux/fs.h:1895 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x552/0x5c0 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_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 3853 Comm: syz-executor.0 Not tainted 5.11.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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