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BUG: KCSAN: data-race in __fat_write_inode / _copy_to_iter

write to 0xffff88811b3a235c of 4 bytes by task 3636 on cpu 0:
 __fat_write_inode+0x23b/0x530 fs/fat/inode.c:886
 fat_write_inode+0xbd/0xd0 fs/fat/inode.c:922
 write_inode fs/fs-writeback.c:1525 [inline]
 __writeback_single_inode+0x340/0x850 fs/fs-writeback.c:1745
 writeback_single_inode+0x16c/0x3f0 fs/fs-writeback.c:1801
 sync_inode_metadata+0x5c/0x90 fs/fs-writeback.c:2871
 __generic_file_fsync+0xed/0x140 fs/libfs.c:1536
 fat_file_fsync+0x46/0x100 fs/fat/file.c:191
 vfs_fsync_range+0x116/0x130 fs/sync.c:187
 generic_write_sync include/linux/fs.h:2970 [inline]
 generic_file_write_iter+0x1c3/0x310 mm/filemap.c:4319
 iter_file_splice_write+0x5f1/0x980 fs/splice.c:743
 do_splice_from fs/splice.c:941 [inline]
 direct_splice_actor+0x160/0x2c0 fs/splice.c:1164
 splice_direct_to_actor+0x302/0x670 fs/splice.c:1108
 do_splice_direct_actor fs/splice.c:1207 [inline]
 do_splice_direct+0xd7/0x150 fs/splice.c:1233
 do_sendfile+0x398/0x660 fs/read_write.c:1363
 __do_sys_sendfile64 fs/read_write.c:1424 [inline]
 __se_sys_sendfile64 fs/read_write.c:1410 [inline]
 __x64_sys_sendfile64+0x110/0x150 fs/read_write.c:1410
 x64_sys_call+0xfbd/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:41
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b3a2200 of 3584 bytes by task 3376 on cpu 1:
 instrument_copy_to_user include/linux/instrumented.h:113 [inline]
 copy_to_user_iter lib/iov_iter.c:24 [inline]
 iterate_ubuf include/linux/iov_iter.h:30 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:300 [inline]
 iterate_and_advance include/linux/iov_iter.h:328 [inline]
 _copy_to_iter+0x130/0xd00 lib/iov_iter.c:185
 copy_page_to_iter+0x171/0x2b0 lib/iov_iter.c:362
 copy_folio_to_iter include/linux/uio.h:198 [inline]
 filemap_read+0x43d/0x8c0 mm/filemap.c:2742
 blkdev_read_iter+0x228/0x2d0 block/fops.c:796
 new_sync_read fs/read_write.c:484 [inline]
 vfs_read+0x5cc/0x6f0 fs/read_write.c:565
 ksys_read+0xe8/0x1b0 fs/read_write.c:708
 __do_sys_read fs/read_write.c:717 [inline]
 __se_sys_read fs/read_write.c:715 [inline]
 __x64_sys_read+0x42/0x50 fs/read_write.c:715
 x64_sys_call+0x2874/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3376 Comm: udevd Not tainted 6.14.0-rc6-syzkaller-00003-g4d872d51bc9d #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
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