KCSAN: data-race in generic_write_end / mpage_process_page_bufs

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: b4bd9343 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=1315c713600000
kernel config: https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80
dashboard link: https://syzkaller.appspot.com/bug?extid=7d49c34deef9c3ddc57f
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+7d49c3...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in generic_write_end / mpage_process_page_bufs

write to 0xffff888125d34a18 of 8 bytes by task 7747 on cpu 0:
i_size_write include/linux/fs.h:875 [inline]
generic_write_end+0xd0/0x1f0 fs/buffer.c:2143
ext4_da_write_end+0x158/0x620 fs/ext4/inode.c:3217
generic_perform_write+0x1d3/0x320 mm/filemap.c:3338
__generic_file_write_iter+0x251/0x380 mm/filemap.c:3456
ext4_file_write_iter+0x1fc/0xa40 fs/ext4/file.c:270
call_write_iter include/linux/fs.h:1870 [inline]
new_sync_write+0x388/0x4a0 fs/read_write.c:483
__vfs_write+0xb1/0xc0 fs/read_write.c:496
vfs_write fs/read_write.c:558 [inline]
vfs_write+0x18a/0x390 fs/read_write.c:542
ksys_write+0xd5/0x1b0 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+0x4c/0x60 fs/read_write.c:620
do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888125d34a18 of 8 bytes by task 7731 on cpu 1:
mpage_process_page_bufs+0x41/0x3d0 fs/ext4/inode.c:2345
mpage_prepare_extent_to_map+0x48b/0x760 fs/ext4/inode.c:2722
ext4_writepages+0xb4f/0x22e0 fs/ext4/inode.c:2894
do_writepages+0x6b/0x170 mm/page-writeback.c:2342
__filemap_fdatawrite_range+0x19e/0x200 mm/filemap.c:415
__filemap_fdatawrite mm/filemap.c:423 [inline]
filemap_flush+0x2d/0x40 mm/filemap.c:450
ext4_alloc_da_blocks+0x78/0x180 fs/ext4/inode.c:3289
ext4_release_file+0x10d/0x200 fs/ext4/file.c:88
__fput+0x1e1/0x520 fs/file_table.c:280
____fput+0x1f/0x30 fs/file_table.c:313
task_work_run+0xf6/0x130 kernel/task_work.c:113
get_signal+0x12c3/0x1320 kernel/signal.c:2528
do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7731 Comm: syz-executor.2 Not tainted 5.3.0+ #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 syzk...@googlegroups.com.

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

[syzbot]

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