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BUG: KCSAN: data-race in mark_buffer_dirty_inode / mark_buffer_dirty_inode

write to 0xffff88811e9b35a0 of 8 bytes by task 15729 on cpu 0:
 mark_buffer_dirty_inode+0x185/0x1b0 fs/buffer.c:585
 fat_mirror_bhs+0x200/0x320 fs/fat/fatent.c:396
 fat_ent_write+0xbf/0xd0 fs/fat/fatent.c:421
 fat_chain_add+0x149/0x400 fs/fat/misc.c:130
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x429/0x5a0 fs/fat/inode.c:190
 __block_write_begin_int+0x4bf/0xfd0 fs/buffer.c:2011
 __block_write_begin fs/buffer.c:2061 [inline]
 block_write_begin+0x4f/0x1c0 fs/buffer.c:2120
 cont_write_begin+0x42c/0x610 fs/buffer.c:2469
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3222
 cont_expand_zero fs/buffer.c:2396 [inline]
 cont_write_begin+0x13f/0x610 fs/buffer.c:2459
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 pagecache_write_begin+0x54/0x70 mm/filemap.c:3222
 generic_cont_expand_simple+0x6d/0x100 fs/buffer.c:2360
 fat_cont_expand+0x3a/0x160 fs/fat/file.c:222
 fat_fallocate+0x180/0x1d0 fs/fat/file.c:300
 vfs_fallocate+0x43c/0x5d0 fs/open.c:309
 ksys_fallocate fs/open.c:332 [inline]
 __do_sys_fallocate fs/open.c:340 [inline]
 __se_sys_fallocate fs/open.c:338 [inline]
 __x64_sys_fallocate+0x75/0xc0 fs/open.c:338
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811e9b35a0 of 8 bytes by task 15740 on cpu 1:
 mark_buffer_dirty_inode+0x92/0x1b0 fs/buffer.c:581
 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:2011
 __block_write_begin fs/buffer.c:2061 [inline]
 block_write_begin+0x4f/0x1c0 fs/buffer.c:2120
 cont_write_begin+0x42c/0x610 fs/buffer.c:2469
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x390 mm/filemap.c:3403
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3532
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3564
 call_write_iter include/linux/fs.h:1882 [inline]
 do_iter_readv_writev+0x32e/0x3d0 fs/read_write.c:713
 do_iter_write+0x112/0x4b0 fs/read_write.c:1018
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1059
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x56a/0xba0 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1601 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 15740 Comm: syz-executor.0 Not tainted 5.9.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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