KCSAN: data-race in wbt_wait / wbt_wait

[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=12fda80f600000
kernel config: https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=ba8947364367f96fe16b
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [ax...@kernel.dk linux...@vger.kernel.org
linux-...@vger.kernel.org el...@google.com]

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+ba8947...@syzkaller.appspotmail.com

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

read to 0xffff88821aa6d140 of 8 bytes by task 10372 on cpu 1:
wb_timestamp block/blk-wbt.c:88 [inline]
wb_timestamp block/blk-wbt.c:83 [inline]
wbt_wait+0x1f9/0x250 block/blk-wbt.c:587
__rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72
rq_qos_throttle block/blk-rq-qos.h:185 [inline]
blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971
generic_make_request block/blk-core.c:1064 [inline]
generic_make_request+0x196/0x740 block/blk-core.c:1006
submit_bio+0x96/0x3c0 block/blk-core.c:1190
submit_bh_wbc+0x40f/0x460 fs/buffer.c:3095
submit_bh fs/buffer.c:3101 [inline]
__bread_slow fs/buffer.c:1177 [inline]
__bread_gfp+0xe7/0x1e0 fs/buffer.c:1359
sb_bread include/linux/buffer_head.h:307 [inline]
fat__get_entry+0x35e/0x4f0 fs/fat/dir.c:100
fat_get_entry fs/fat/dir.c:128 [inline]
fat_get_short_entry+0x103/0x200 fs/fat/dir.c:877
fat_subdirs+0x6b/0x110 fs/fat/dir.c:943
fat_read_root fs/fat/inode.c:1416 [inline]
fat_fill_super+0x1552/0x1f50 fs/fat/inode.c:1862
vfat_fill_super+0x3b/0x50 fs/fat/namei_vfat.c:1050
mount_bdev+0x262/0x2d0 fs/super.c:1415
vfat_mount+0x3e/0x60 fs/fat/namei_vfat.c:1057

write to 0xffff88821aa6d140 of 8 bytes by task 10375 on cpu 0:
wb_timestamp block/blk-wbt.c:89 [inline]
wb_timestamp block/blk-wbt.c:83 [inline]
wbt_wait+0x21e/0x250 block/blk-wbt.c:587
__rq_qos_throttle+0x47/0x70 block/blk-rq-qos.c:72
rq_qos_throttle block/blk-rq-qos.h:185 [inline]
blk_mq_make_request+0x29c/0xf60 block/blk-mq.c:1971
generic_make_request block/blk-core.c:1064 [inline]
generic_make_request+0x196/0x740 block/blk-core.c:1006
submit_bio+0x96/0x3c0 block/blk-core.c:1190
mpage_bio_submit fs/mpage.c:66 [inline]
mpage_readpages+0x36c/0x3c0 fs/mpage.c:410
blkdev_readpages+0x36/0x50 fs/block_dev.c:620
read_pages+0xa2/0x2d0 mm/readahead.c:126
__do_page_cache_readahead+0x353/0x390 mm/readahead.c:212
force_page_cache_readahead+0x13a/0x1f0 mm/readahead.c:243
page_cache_sync_readahead+0x1cf/0x1e0 mm/readahead.c:522
generic_file_buffered_read mm/filemap.c:2050 [inline]
generic_file_read_iter+0xeb6/0x1440 mm/filemap.c:2323
blkdev_read_iter+0xb2/0xe0 fs/block_dev.c:2010

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10375 Comm: blkid 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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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.