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BUG: KCSAN: data-race in copy_folio_from_iter_atomic / fat12_ent_put

write to 0xffff888121f76082 of 1 bytes by task 6014 on cpu 1:
 fat12_ent_put+0x74/0x180 fs/fat/fatent.c:168
 fat_alloc_clusters+0x55e/0xc40 fs/fat/fatent.c:508
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x252/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
 block_write_begin fs/buffer.c:2234 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2524 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2586
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:305
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 x64_sys_call+0x298e/0x3020 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888121f76000 of 512 bytes by task 49 on cpu 0:
 memcpy_from_iter lib/iov_iter.c:85 [inline]
 iterate_bvec include/linux/iov_iter.h:123 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:306 [inline]
 iterate_and_advance include/linux/iov_iter.h:330 [inline]
 __copy_from_iter lib/iov_iter.c:261 [inline]
 copy_folio_from_iter_atomic+0x728/0x10a0 lib/iov_iter.c:491
 generic_perform_write+0x2c4/0x490 mm/filemap.c:4376
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3414
 lo_rw_aio+0x6a9/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1921 [inline]
 loop_process_work+0x567/0xac0 drivers/block/loop.c:1956
 loop_workfn+0x31/0x40 drivers/block/loop.c:1980
 process_one_work kernel/workqueue.c:3322 [inline]
 process_scheduled_works+0x4d4/0x9a0 kernel/workqueue.c:3405
 worker_thread+0x569/0x750 kernel/workqueue.c:3486
 kthread+0x221/0x270 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 49 Comm: kworker/u8:3 Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: loop6 loop_workfn
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