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BUG: KCSAN: data-race in copy_folio_from_iter_atomic / fat16_ent_put

write to 0xffff888106d388da of 2 bytes by task 3931 on cpu 1:
 fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
 fat_alloc_clusters+0x4ce/0xa80 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:107 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x3fd/0xf90 fs/buffer.c:2151
 block_write_begin fs/buffer.c:2262 [inline]
 cont_write_begin+0x5fc/0x970 fs/buffer.c:2601
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_perform_write+0x184/0x490 mm/filemap.c:4112
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4229
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4255
 aio_write+0x2e2/0x410 fs/aio.c:1634
 __io_submit_one fs/aio.c:-1 [inline]
 io_submit_one+0xacd/0x11d0 fs/aio.c:2053
 __do_sys_io_submit fs/aio.c:2112 [inline]
 __se_sys_io_submit+0xfb/0x280 fs/aio.c:2082
 __x64_sys_io_submit+0x43/0x50 fs/aio.c:2082
 x64_sys_call+0xfc4/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:210
 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 0xffff888106d38800 of 512 bytes by task 31 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:4130
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3463
 lo_rw_aio+0x6ed/0x7a0 include/linux/fs.h:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1889 [inline]
 loop_process_work+0x52d/0xa60 drivers/block/loop.c:1924
 loop_workfn+0x31/0x40 drivers/block/loop.c:1948
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3321
 worker_thread+0x582/0x770 kernel/workqueue.c:3402
 kthread+0x486/0x510 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: 31 Comm: kworker/u8:1 Not tainted 6.16.0-rc2-syzkaller-00087-g24770983ccfe #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Workqueue: loop3 loop_workfn
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