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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_recalc_inode

write to 0xffff88809ec59bb8 of 8 bytes by task 15676 on cpu 1:
 shmem_add_to_page_cache+0x4b0/0x6c0 mm/shmem.c:643
 shmem_getpage_gfp+0x43f/0x16c0 mm/shmem.c:1871
 shmem_getpage mm/shmem.c:154 [inline]
 shmem_write_begin+0x98/0x130 mm/shmem.c:2483
 generic_perform_write+0x13a/0x320 mm/filemap.c:3302
 __generic_file_write_iter+0x240/0x370 mm/filemap.c:3431
 generic_file_write_iter+0x294/0x38e mm/filemap.c:3463
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:484
 __vfs_write+0x9e/0xb0 fs/read_write.c:497
 vfs_write fs/read_write.c:559 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:543
 ksys_pwrite64+0xe4/0x110 fs/read_write.c:666
 __do_sys_pwrite64 fs/read_write.c:676 [inline]
 __se_sys_pwrite64 fs/read_write.c:673 [inline]
 __x64_sys_pwrite64+0x5b/0x70 fs/read_write.c:673
 do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88809ec59bb8 of 8 bytes by task 15685 on cpu 0:
 shmem_recalc_inode+0x50/0x180 mm/shmem.c:306
 shmem_getpage_gfp+0x5ca/0x16c0 mm/shmem.c:1885
 shmem_getpage mm/shmem.c:154 [inline]
 shmem_file_read_iter+0x396/0x850 mm/shmem.c:2560
 call_read_iter include/linux/fs.h:1901 [inline]
 do_iter_readv_writev+0x570/0x5d0 fs/read_write.c:692
 do_iter_read+0x1e6/0x3a0 fs/read_write.c:934
 vfs_iter_read+0x56/0x80 fs/read_write.c:976
 lo_read_simple drivers/block/loop.c:353 [inline]
 do_req_filebacked drivers/block/loop.c:628 [inline]
 loop_handle_cmd drivers/block/loop.c:1974 [inline]
 loop_queue_work+0x876/0x14b9 drivers/block/loop.c:1991
 kthread_worker_fn+0x1d0/0x3c0 kernel/kthread.c:684
 loop_kthread_worker_fn+0x49/0x60 drivers/block/loop.c:923
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 15685 Comm: loop1 Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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