KCSAN: data-race in fat16_ent_put / fat_search_long

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
git tree:       https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=14ccaac8e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=11010f0000e50c63c2cc
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+11010f0000e50c63c2cc@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KCSAN: data-race in fat16_ent_put / fat_search_long

write to 0xffff8880a209c96a of 2 bytes by task 11985 on cpu 0:
 fat16_ent_put+0x5b/0x90 fs/fat/fatent.c:181
 fat_ent_write+0x6d/0xf0 fs/fat/fatent.c:415
 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
 pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148
 cont_expand_zero fs/buffer.c:2353 [inline]
 cont_write_begin+0x17a/0x670 fs/buffer.c:2416
 fat_write_begin+0x72/0xc0 fs/fat/inode.c:235
 pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148
 generic_cont_expand_simple+0xb0/0x120 fs/buffer.c:2317

read to 0xffff8880a209c96b of 1 bytes by task 11990 on cpu 1:
 fat_search_long+0x20a/0xc60 fs/fat/dir.c:484
 vfat_find+0xc1/0xd0 fs/fat/namei_vfat.c:698
 vfat_lookup+0x75/0x350 fs/fat/namei_vfat.c:712
 lookup_open fs/namei.c:3203 [inline]
 do_last fs/namei.c:3314 [inline]
 path_openat+0x15b6/0x36e0 fs/namei.c:3525
 do_filp_open+0x11e/0x1b0 fs/namei.c:3555
 do_sys_open+0x3b3/0x4f0 fs/open.c:1097
 __do_sys_open fs/open.c:1115 [inline]
 __se_sys_open fs/open.c:1110 [inline]
 __x64_sys_open+0x55/0x70 fs/open.c:1110
 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: 11990 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
==================================================================


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