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SubjectKCSAN: data-race in __fat_write_inode / fat12_ent_get
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Hello,

syzbot found the following crash on:

HEAD commit: 40959e34 kcsan: Avoid blocking producers in prepare_report()
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=1201d5a3e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=1ab2c758651b11f6
dashboard link: https://syzkaller.appspot.com/bug?extid=6f1624f937d9d6911e2d
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+6f1624f937d9d6911e2d@syzkaller.appspotmail.com

FAT-fs (loop1): error, clusters badly computed (876 != 875)
FAT-fs (loop1): error, clusters badly computed (877 != 876)
FAT-fs (loop1): error, clusters badly computed (878 != 877)
==================================================================
BUG: KCSAN: data-race in __fat_write_inode / fat12_ent_get

write to 0xffff8881015f423c of 4 bytes by task 9966 on cpu 1:
__fat_write_inode+0x246/0x510 fs/fat/inode.c:877
fat_write_inode+0x67/0xe0 fs/fat/inode.c:909
write_inode fs/fs-writeback.c:1312 [inline]
__writeback_single_inode+0x722/0x910 fs/fs-writeback.c:1511
writeback_single_inode+0x219/0x2f0 fs/fs-writeback.c:1565
sync_inode fs/fs-writeback.c:2602 [inline]
sync_inode_metadata+0x75/0xa0 fs/fs-writeback.c:2622
__generic_file_fsync+0x117/0x180 fs/libfs.c:1081
fat_file_fsync+0x54/0x120 fs/fat/file.c:190
vfs_fsync_range+0x7c/0x150 fs/sync.c:197
generic_write_sync include/linux/fs.h:2867 [inline]
generic_file_write_iter+0x31c/0x38e mm/filemap.c:3452
call_write_iter include/linux/fs.h:1901 [inline]
do_iter_readv_writev+0x4a7/0x5d0 fs/read_write.c:693
do_iter_write fs/read_write.c:998 [inline]
do_iter_write+0x137/0x3a0 fs/read_write.c:979
vfs_iter_write+0x56/0x80 fs/read_write.c:1039
iter_file_splice_write+0x530/0x830 fs/splice.c:760
do_splice_from fs/splice.c:863 [inline]
direct_splice_actor+0x97/0xb0 fs/splice.c:1037
splice_direct_to_actor+0x22f/0x540 fs/splice.c:992
do_splice_direct+0x152/0x1d0 fs/splice.c:1080
do_sendfile+0x396/0x810 fs/read_write.c:1520
__do_sys_sendfile64 fs/read_write.c:1575 [inline]
__se_sys_sendfile64 fs/read_write.c:1567 [inline]
__x64_sys_sendfile64+0xb8/0x140 fs/read_write.c:1567
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881015f423d of 1 bytes by task 9960 on cpu 0:
fat12_ent_get+0x5e/0x120 fs/fat/fatent.c:125
fat_ent_read+0x3de/0x560 fs/fat/fatent.c:370
fat_get_cluster+0x52b/0x920 fs/fat/cache.c:266
fat_bmap_cluster fs/fat/cache.c:299 [inline]
fat_get_mapped_cluster+0x105/0x230 fs/fat/cache.c:320
fat_bmap+0x146/0x28e fs/fat/cache.c:384
__fat_get_block fs/fat/inode.c:165 [inline]
fat_get_block+0x244/0x4f0 fs/fat/inode.c:190
__block_write_begin_int+0x306/0xf80 fs/buffer.c:2008
__block_write_begin fs/buffer.c:2058 [inline]
block_write_begin+0x76/0x160 fs/buffer.c:2117
cont_write_begin+0x3bd/0x660 fs/buffer.c:2466
fat_write_begin+0x69/0xc0 fs/fat/inode.c:236
pagecache_write_begin+0x67/0x90 mm/filemap.c:3106
cont_expand_zero fs/buffer.c:2393 [inline]
cont_write_begin+0x176/0x660 fs/buffer.c:2456
fat_write_begin+0x69/0xc0 fs/fat/inode.c:236
generic_perform_write+0x13a/0x320 mm/filemap.c:3287
__generic_file_write_iter+0x240/0x370 mm/filemap.c:3416
generic_file_write_iter+0x294/0x38e mm/filemap.c:3448
call_write_iter include/linux/fs.h:1901 [inline]
new_sync_write+0x303/0x400 fs/read_write.c:483
__vfs_write+0x9e/0xb0 fs/read_write.c:496
vfs_write fs/read_write.c:558 [inline]
vfs_write+0x189/0x380 fs/read_write.c:542
ksys_write+0xc5/0x1a0 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+0x49/0x60 fs/read_write.c:620
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9960 Comm: syz-executor.1 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

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