==================================================================
BUG: KCSAN: data-race in mark_buffer_dirty_inode / mark_buffer_dirty_inode

write to 0xffff888105df1c88 of 8 bytes by task 4959 on cpu 1:
 mark_buffer_dirty_inode+0x18b/0x1d0 fs/buffer.c:585
 fat_mirror_bhs+0x205/0x330 fs/fat/fatent.c:396
 fat_alloc_clusters+0x95e/0xa50 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+0x4a2/0x1060 fs/buffer.c:2007
 __block_write_begin fs/buffer.c:2057 [inline]
 block_write_begin fs/buffer.c:2117 [inline]
 cont_write_begin+0x522/0x850 fs/buffer.c:2466
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3676
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3805
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3837
 call_write_iter include/linux/fs.h:2116 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x773/0xda0 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1325 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1311
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888105df1c88 of 8 bytes by task 4940 on cpu 0:
 mark_buffer_dirty_inode+0x92/0x1d0 fs/buffer.c:581
 fat_mirror_bhs+0x205/0x330 fs/fat/fatent.c:396
 fat_alloc_clusters+0x95e/0xa50 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+0x4a2/0x1060 fs/buffer.c:2007
 __block_write_begin fs/buffer.c:2057 [inline]
 block_write_begin fs/buffer.c:2117 [inline]
 cont_write_begin+0x522/0x850 fs/buffer.c:2466
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3676
 __generic_file_write_iter+0x161/0x300 mm/filemap.c:3805
 generic_file_write_iter+0x75/0x130 mm/filemap.c:3837
 call_write_iter include/linux/fs.h:2116 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x773/0xda0 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1325 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1311
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4940 Comm: syz-executor.3 Not tainted 5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================
syz-executor.3 (4940) used greatest stack depth: 10576 bytes left