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BUG: KCSAN: data-race in shmem_file_read_iter / shmem_write_end

write to 0xffff88814cbe9270 of 8 bytes by task 30861 on cpu 1:
 i_size_write include/linux/fs.h:932 [inline]
 shmem_write_end+0x9a/0x470 mm/shmem.c:2741
 generic_perform_write+0x267/0x410 mm/filemap.c:3941
 shmem_file_write_iter+0xc8/0xe0 mm/shmem.c:2893
 do_iter_readv_writev+0x2b0/0x330
 vfs_writev+0x2e9/0x860 fs/read_write.c:971
 do_pwritev fs/read_write.c:1072 [inline]
 __do_sys_pwritev fs/read_write.c:1119 [inline]
 __se_sys_pwritev fs/read_write.c:1114 [inline]
 __x64_sys_pwritev+0x100/0x1c0 fs/read_write.c:1114
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

read to 0xffff88814cbe9270 of 8 bytes by task 7995 on cpu 0:
 i_size_read include/linux/fs.h:910 [inline]
 shmem_file_read_iter+0x96/0x550 mm/shmem.c:2777
 do_iter_readv_writev+0x2b0/0x330
 vfs_iter_read+0x172/0x390 fs/read_write.c:830
 lo_read_simple drivers/block/loop.c:286 [inline]
 do_req_filebacked drivers/block/loop.c:496 [inline]
 loop_handle_cmd drivers/block/loop.c:1909 [inline]
 loop_process_work+0x7a1/0x10e0 drivers/block/loop.c:1944
 loop_workfn+0x31/0x40 drivers/block/loop.c:1968
 process_one_work kernel/workqueue.c:2633 [inline]
 process_scheduled_works+0x5b8/0xa40 kernel/workqueue.c:2706
 worker_thread+0x525/0x730 kernel/workqueue.c:2787
 kthread+0x1d7/0x210 kernel/kthread.c:388
 ret_from_fork+0x48/0x60 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242

value changed: 0x0000000000dcf000 -> 0x0000000000dd1000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7995 Comm: kworker/u4:18 Not tainted 6.8.0-rc3-syzkaller-00317-g7521f258ea30 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
Workqueue: loop0 loop_workfn
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