==================================================================
BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88813583a05c of 4 bytes by task 9010 on cpu 1:
 __fat_write_inode+0x236/0x520 fs/fat/inode.c:-1
 fat_write_inode+0xbf/0xd0 fs/fat/inode.c:924
 write_inode fs/fs-writeback.c:1525 [inline]
 __writeback_single_inode+0x319/0x7c0 fs/fs-writeback.c:1745
 writeback_single_inode+0x167/0x3e0 fs/fs-writeback.c:1801
 sync_inode_metadata+0x5b/0x90 fs/fs-writeback.c:2883
 writeback_inode fs/fat/inode.c:1870 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1890
 fat_truncate_blocks+0x4d8/0x550 fs/fat/file.c:395
 fat_write_failed fs/fat/inode.c:218 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:246
 generic_perform_write+0x30f/0x490 mm/filemap.c:4196
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4292
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4318
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x4a0/0x8e0 fs/read_write.c:686
 ksys_write+0xda/0x1a0 fs/read_write.c:738
 __do_sys_write fs/read_write.c:749 [inline]
 __se_sys_write fs/read_write.c:746 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:746
 x64_sys_call+0x27fe/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:2
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88813583a000 of 2048 bytes by task 3657 on cpu 0:
 memcpy_from_iter lib/iov_iter.c:73 [inline]
 iterate_bvec include/linux/iov_iter.h:123 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:304 [inline]
 iterate_and_advance include/linux/iov_iter.h:328 [inline]
 __copy_from_iter lib/iov_iter.c:249 [inline]
 copy_folio_from_iter_atomic+0x7fc/0x1170 lib/iov_iter.c:479
 generic_perform_write+0x2c2/0x490 mm/filemap.c:4193
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3499
 lo_rw_aio+0x6a0/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1910 [inline]
 loop_process_work+0x52d/0xa60 drivers/block/loop.c:1945
 loop_workfn+0x31/0x40 drivers/block/loop.c:1969
 process_one_work kernel/workqueue.c:3236 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3319
 worker_thread+0x582/0x770 kernel/workqueue.c:3400
 kthread+0x489/0x510 kernel/kthread.c:464
 ret_from_fork+0xdd/0x150 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3657 Comm: kworker/u8:21 Not tainted 6.16.0-syzkaller-11322-g352af6a011d5 #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue: loop7 loop_workfn
==================================================================