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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff88810a37a828 of 8 bytes by task 20583 on cpu 1:
 shmem_add_to_page_cache+0x5cd/0x6e0 mm/shmem.c:718
 shmem_getpage_gfp+0x86a/0x1980 mm/shmem.c:1929
 shmem_getpage mm/shmem.c:155 [inline]
 shmem_file_read_iter+0x1b8/0x760 mm/shmem.c:2607
 call_read_iter include/linux/fs.h:1971 [inline]
 do_iter_readv_writev+0x2e8/0x360 fs/read_write.c:738
 do_iter_read+0x109/0x4b0 fs/read_write.c:801
 vfs_iter_read+0x4c/0x70 fs/read_write.c:843
 lo_read_simple drivers/block/loop.c:360 [inline]
 do_req_filebacked drivers/block/loop.c:636 [inline]
 loop_handle_cmd drivers/block/loop.c:2045 [inline]
 loop_queue_work+0xdfb/0x1330 drivers/block/loop.c:2063
 kthread_worker_fn+0x28f/0x540 kernel/kthread.c:735
 loop_kthread_worker_fn+0x3e/0x50 drivers/block/loop.c:932
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88810a37a828 of 8 bytes by task 20578 on cpu 0:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0xba0/0x1980 mm/shmem.c:1939
 shmem_getpage mm/shmem.c:155 [inline]
 shmem_write_begin+0x83/0x100 mm/shmem.c:2530
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3575
 __generic_file_write_iter+0x15c/0x340 mm/filemap.c:3704
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3736
 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_writev fs/read_write.c:939 [inline]
 do_pwritev+0x210/0x410 fs/read_write.c:1036
 __do_sys_pwritev fs/read_write.c:1083 [inline]
 __se_sys_pwritev fs/read_write.c:1078 [inline]
 __x64_sys_pwritev+0x54/0x60 fs/read_write.c:1078
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20578 Comm: syz-executor.5 Not tainted 5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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