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BUG: KCSAN: data-race in __fat_fs_error / fat_mirror_bhs

write to 0xffff88808a151050 of 8 bytes by task 21395 on cpu 0:
 __fat_fs_error+0x151/0x198 fs/fat/misc.c:38
 fat_get_cluster+0x902/0x980 fs/fat/cache.c:270
 fat_chain_add+0x250/0x400 fs/fat/misc.c:115
 fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189
 __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968
 __block_write_begin fs/buffer.c:2018 [inline]
 block_write_begin+0x77/0x160 fs/buffer.c:2077
 cont_write_begin+0x3d6/0x670 fs/buffer.c:2426
 fat_write_begin+0x72/0xc0 fs/fat/inode.c:235
 generic_perform_write+0x136/0x320 mm/filemap.c:3305
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3434
 generic_file_write_iter+0x28c/0x390 mm/filemap.c:3466
 call_write_iter include/linux/fs.h:1895 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88808a151050 of 8 bytes by task 21404 on cpu 1:
 fat_mirror_bhs+0x108/0x2f0 fs/fat/fatent.c:397
 fat_ent_write+0x9f/0xf0 fs/fat/fatent.c:421
 fat_chain_add+0x34e/0x400 fs/fat/misc.c:130
 fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189
 __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968
 __block_write_begin fs/buffer.c:2018 [inline]
 block_write_begin+0x77/0x160 fs/buffer.c:2077
 cont_write_begin+0x3d6/0x670 fs/buffer.c:2426
 fat_write_begin+0x72/0xc0 fs/fat/inode.c:235
 generic_perform_write+0x136/0x320 mm/filemap.c:3305
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3434
 generic_file_write_iter+0x28c/0x390 mm/filemap.c:3466
 call_write_iter include/linux/fs.h:1895 [inline]
 aio_write+0x1de/0x2d0 fs/aio.c:1583
 __io_submit_one fs/aio.c:1815 [inline]
 io_submit_one+0x639/0xe70 fs/aio.c:1862
 __do_sys_io_submit fs/aio.c:1921 [inline]
 __se_sys_io_submit fs/aio.c:1891 [inline]
 __x64_sys_io_submit+0x104/0x2d0 fs/aio.c:1891
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 21404 Comm: syz-executor.2 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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