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BUG: KCSAN: data-race in mark_buffer_dirty_inode / mark_buffer_dirty_inode

write to 0xffff88808989cc88 of 8 bytes by task 14776 on cpu 0:
 mark_buffer_dirty_inode+0x185/0x1b0 fs/buffer.c:586
 fat_mirror_bhs+0x200/0x320 fs/fat/fatent.c:396
 fat_alloc_clusters+0x95e/0xa60 fs/fat/fatent.c:541
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x213/0x5a0 fs/fat/inode.c:190
 __block_write_begin_int+0x4bf/0xfd0 fs/buffer.c:2012
 __block_write_begin fs/buffer.c:2062 [inline]
 block_write_begin+0x4f/0x190 fs/buffer.c:2121
 cont_write_begin+0x42c/0x610 fs/buffer.c:2470
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x390 mm/filemap.c:3318
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3447
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3479
 call_write_iter include/linux/fs.h:1908 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x607/0x690 fs/read_write.c:578
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88808989cc88 of 8 bytes by task 14778 on cpu 1:
 mark_buffer_dirty_inode+0x92/0x1b0 fs/buffer.c:582
 fat_mirror_bhs+0x200/0x320 fs/fat/fatent.c:396
 fat_alloc_clusters+0x95e/0xa60 fs/fat/fatent.c:541
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x213/0x5a0 fs/fat/inode.c:190
 __block_write_begin_int+0x4bf/0xfd0 fs/buffer.c:2012
 __block_write_begin fs/buffer.c:2062 [inline]
 block_write_begin+0x4f/0x190 fs/buffer.c:2121
 cont_write_begin+0x42c/0x610 fs/buffer.c:2470
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3137
 cont_expand_zero fs/buffer.c:2397 [inline]
 cont_write_begin+0x13f/0x610 fs/buffer.c:2460
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3137
 generic_cont_expand_simple+0x6d/0x100 fs/buffer.c:2361
 fat_cont_expand+0x3a/0x160 fs/fat/file.c:222
 fat_setattr+0x229/0x760 fs/fat/file.c:501
 notify_change+0x7d4/0xa20 fs/attr.c:336
 do_truncate fs/open.c:64 [inline]
 do_sys_ftruncate+0x39f/0x3e0 fs/open.c:195
 __do_sys_ftruncate fs/open.c:205 [inline]
 __se_sys_ftruncate fs/open.c:203 [inline]
 __x64_sys_ftruncate+0x2f/0x40 fs/open.c:203
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 14778 Comm: syz-executor.2 Not tainted 5.8.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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