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BUG: KCSAN: data-race in loop_configure / shmem_write_begin

write to 0xffff88812fd57c70 of 4 bytes by task 22704 on cpu 1:
 mapping_set_gfp_mask include/linux/pagemap.h:126 [inline]
 loop_configure+0x7df/0xda0 drivers/block/loop.c:1221
 lo_ioctl+0x555/0x11f0 drivers/block/loop.c:1783
 blkdev_ioctl+0x1d0/0x3c0 block/ioctl.c:585
 block_ioctl+0x6d/0x80 fs/block_dev.c:1662
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:1069 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:1055
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:1055
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88812fd57c70 of 4 bytes by task 22699 on cpu 0:
 mapping_gfp_mask include/linux/pagemap.h:110 [inline]
 shmem_getpage mm/shmem.c:156 [inline]
 shmem_write_begin+0x62/0x100 mm/shmem.c:2538
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3660
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3789
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3821
 call_write_iter include/linux/fs.h:2114 [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+0x1e5/0x3a0 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_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x01100cca -> 0x01100c0a

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 22699 Comm: syz-executor.2 Not tainted 5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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