umount failed when format without partition

[gongzhifeng]

I'm using exfat 1.2.3 on soc board with microSD card.
Format card with command "mkfs.exfat /dev/mmcblk0", and the mount point is "/sdcard",
"umount /sdcard" will block. I try to trace the reason, it's blocked in system call "umount", and trace into kernel space, it's blocked in "fs/fuse/dev.c" wait_event_interruptible function.
But format with "mkfs.exfat /dev/mmcblk0p1", then "umount /sdcard" is ok.
So, is this a bug, which can't handle mmcblk0 umount?

[relan]

fuse-exfat should work with any block device. It makes no difference between a whole device and a partition. Try to attach to the mount.exfat-fuse process to see where it hangs. BTW which kernel and libc versions are used on this board?

[gongzhifeng]

kernel 3.10.73+
libc 2.19-2014.08
gcc version 4.9.2 20140904
When umount hang, the mount.exfat-fuse process is still alive. Coud you tell me how to attach the mount.exfat-fuse process?
And there's no such issue when using "mkfs.vfat /dev/mmcblk0".
Coud you reproduce the issue?

[relan]

Coud you tell me how to attach the mount.exfat-fuse process?

gdb -p PID

[gongzhifeng]

I tried, but there's no detail information.
gdb attach in a console, umount in another console:

ps x

338 ? Ss 0:00 mount.exfat /dev/mmcblk0 /sdcard -o errors=remount-ro

gdb -p 338

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Attaching to process 339
Reading symbols from /usr/sbin/mount.exfat-fuse...done.
Reading symbols from /usr/lib/libfuse.so.2...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libfuse.so.2
Reading symbols from /lib/libpthread.so.0...done.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/libthread_db.so.1".
Loaded symbols for /lib/libpthread.so.0
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/ld-linux-armhf.so.3...done.
Loaded symbols for /lib/ld-linux-armhf.so.3
0x76ea3004 in read () at ../sysdeps/unix/syscall-template.S:81
81 ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) c
Continuing.

[gongzhifeng]

Maybe it hung into libfuse, but libfuse is stripped, no more debug info.
Reading symbols from /usr/lib/libfuse.so.2...(no debugging symbols found)...done.
Using "strace umount /sdcard", the log show hung "umount(...)" system_call.

[relan]

I can't debug this issue for you. I can only give some advice:

1. Get the stack trace of mount.exfat while it hangs.
2. Look for I/O errors in the kernel log. They may be the reason of problems.

[gongzhifeng]

I try to get the stack trace of mount.exfat, but when "umount" cmd hangs, the gdb also hangs, "Ctrl+C" still can't get interrupt signal, so can't get backtrace.
Maybe it is kernel hang partially, so I trace the kernel, it is blocked in

version linux-3.10
file: kernel/linux/fs/fuse/dev.c
function: wait_answer_interruptible
blocked line: wait_event_interruptible

but no I/O errors in kernel at all.
If this is not exfat bug, where can I report or get help?
Thank you very much!

[relan]

It's very strange that thread hangs in wait_answer_interruptible. Looks like a deadlock. Try to contact board's vendor.

BTW you can run mount.exfat-fuse under strace to see the last syscall before the hang.

Does unmounting of other FUSE-based filesystems (like ntfs-3g) work?

[gongzhifeng]

BTW you can run mount.exfat-fuse under strace to see the last syscall before the hang.

when insert sdcard, mount.exfat-fuse process will auto run, and fork child process.
So I run strace -p $PID -f, the last syscall is close before the hang. the log is
# strace -p 384 -f
Process 384 attached
read(4, "8\0\0\0\3\0\0\0\2\0\0\0\0\0\0\0\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 135168) = 56
writev(4, [{"x\0\0\0\0\0\0\0\2\0\0\0\0\0\0\0", 16},{"\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1\0\0\0\0\0\0\0\0\200\0\0\0\0\0\0"..., 104}], 2) = 120
read(4, "0\0\0\0\2\0\0\0\3\0\0\0\0\0\0\0\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 135168) = 48
read(4, "(\0\0\0&\0\0\0\4\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 135168) = 40
pwrite(3, "\353v\220EXFAT \0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512, 0) = 512
fsync(3) = 0
close(3

Does this hang happen only with fuse-exfat? Does unmounting of native filesystems (like ext4 or f2fs) work? Do other FUSE-based filesystems (like ntfs-3g) work?

I didn't try ext4 or f2fs because of tools not ready, but try vfat on board, umount is ok.
And the fuse-exfat umount is ok on PC Ubuntu 14.04.4 LTS.

[relan]

From this strace output we can see that mount.exfat-fuse hangs on closing /dev/mmcblk0. Source code is

void exfat_unmount(struct exfat* ef)
{
    ...
    finalize_super_block(ef);    <<< pwrite and fsync
    exfat_close(ef->dev);        <<< close
    ...
}

I'd say it's a kernel bug given that mount.exfat-fuse does not hang when it closes /dev/mmcblk0p1. For mount.exfat-fuse there is absolutely no difference between a partition and a whole device, in both cases it works in exactly the same way.

[relan]

No replies in 7 months, closing. Feel free to reopen if you have any new info.