KCSAN: data-race in atime_needs_update / generic_update_time

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 766d004d mm, kcsan: Instrument SLAB free with ASSERT_EXCLU..
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=1543b3ede00000
kernel config: https://syzkaller.appspot.com/x/.config?x=b313b4492ea15bae
dashboard link: https://syzkaller.appspot.com/bug?extid=101664e0742b90681ada
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk 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+101664...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in atime_needs_update / generic_update_time

read to 0xffff88812a5daa40 of 8 bytes by task 13331 on cpu 0:
relatime_need_update fs/inode.c:1647 [inline]
atime_needs_update+0x264/0x360 fs/inode.c:1732
touch_atime+0x63/0x150 fs/inode.c:1747
file_accessed include/linux/fs.h:2220 [inline]
generic_file_buffered_read mm/filemap.c:2236 [inline]
generic_file_read_iter+0x1167/0x1490 mm/filemap.c:2302
blkdev_read_iter+0xae/0xe0 fs/block_dev.c:2039
call_read_iter include/linux/fs.h:1895 [inline]
new_sync_read+0x306/0x450 fs/read_write.c:414
__vfs_read+0x9e/0xb0 fs/read_write.c:427
vfs_read fs/read_write.c:461 [inline]
vfs_read+0x13e/0x2b0 fs/read_write.c:446
ksys_read+0xc5/0x1a0 fs/read_write.c:587
__do_sys_read fs/read_write.c:597 [inline]
__se_sys_read fs/read_write.c:595 [inline]
__x64_sys_read+0x49/0x60 fs/read_write.c:595
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff88812a5daa40 of 16 bytes by task 13249 on cpu 1:
generic_update_time+0xc2/0x270 fs/inode.c:1672
update_time+0x7d/0x90 fs/inode.c:1694
file_update_time+0x205/0x2d0 fs/inode.c:1909
__generic_file_write_iter+0x124/0x370 mm/filemap.c:3366
blkdev_write_iter fs/block_dev.c:2019 [inline]
blkdev_write_iter+0x19c/0x2b0 fs/block_dev.c:1993
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:1581 [inline]
__se_sys_sendfile64 fs/read_write.c:1567 [inline]
__x64_sys_sendfile64+0x121/0x140 fs/read_write.c:1567
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: 1 PID: 13249 Comm: syz-executor.0 Not tainted 5.6.0-rc1-syzkaller #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.