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BUG: KCSAN: data-race in next_uptodate_page / shmem_write_end

write to 0xffff888128752b60 of 8 bytes by task 26041 on cpu 1:
 i_size_write include/linux/fs.h:872 [inline]
 shmem_write_end+0x60/0x3d0 mm/shmem.c:2541
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3593
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3711
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3743
 call_write_iter include/linux/fs.h:1977 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x773/0xda0 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1319 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1311
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888128752b60 of 8 bytes by task 26048 on cpu 0:
 i_size_read include/linux/fs.h:850 [inline]
 next_uptodate_page+0x323/0x670 mm/filemap.c:3068
 first_map_page mm/filemap.c:3085 [inline]
 filemap_map_pages+0xe5/0x870 mm/filemap.c:3111
 do_fault_around mm/memory.c:3920 [inline]
 do_read_fault mm/memory.c:3934 [inline]
 do_fault mm/memory.c:4067 [inline]
 handle_pte_fault mm/memory.c:4318 [inline]
 __handle_mm_fault mm/memory.c:4453 [inline]
 handle_mm_fault+0xda3/0x1a70 mm/memory.c:4551
 do_user_addr_fault+0x60c/0xc00 arch/x86/mm/fault.c:1390
 handle_page_fault arch/x86/mm/fault.c:1475 [inline]
 exc_page_fault+0x94/0x230 arch/x86/mm/fault.c:1531
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:577
 do_strnlen_user lib/strnlen_user.c:49 [inline]
 strnlen_user+0x122/0x1c0 lib/strnlen_user.c:113
 strndup_user+0x1d/0x120 mm/util.c:224
 __do_sys_fsopen fs/fsopen.c:128 [inline]
 __se_sys_fsopen+0x75/0x200 fs/fsopen.c:115
 __x64_sys_fsopen+0x2d/0x40 fs/fsopen.c:115
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26048 Comm: syz-executor.0 Tainted: G        W         5.12.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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