Closing unused descriptors

[Jean-Christian de Rivaz]

Hello,

While trying to reproduce a bug by power cycling a USB modem 4 time per 
hour, I faced this message from the kernel:

cdc_acm 1-2:1.0: no more free acm devices

I found the the problem is caused by ModemManager that still have file 
descriptors open on old ports that was deleted, preventing the kernel to 
recycle the port numbers:

ls -al /proc/$(pgrep ModemManager)/fd
total 0
dr-x------ 2 root root  0 Sep 24 01:44 .
dr-xr-xr-x 7 root root  0 Sep 24 01:44 ..
lr-x------ 1 root root 64 Sep 24 01:44 0 -> /dev/null
lrwx------ 1 root root 64 Sep 24 01:44 1 -> socket:[218296]
lrwx------ 1 root root 64 Sep 24 01:44 10 -> /dev/ttyACM4 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 11 -> /dev/ttyACM5 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 12 -> /dev/ttyACM6 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 13 -> /dev/ttyACM7 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 14 -> /dev/ttyACM8 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 15 -> /dev/ttyACM11 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 16 -> /dev/ttyACM10 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 17 -> /dev/ttyACM12 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 18 -> /dev/ttyACM13 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 19 -> /dev/ttyACM14 (deleted)
l-wx------ 1 root root 64 Sep 24 01:44 2 -> /dev/null
lrwx------ 1 root root 64 Sep 24 01:44 20 -> /dev/ttyACM15 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 21 -> /dev/ttyACM16 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 22 -> /dev/ttyACM17 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 23 -> /dev/ttyACM18 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 24 -> /dev/ttyACM19 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 25 -> /dev/ttyACM20 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 26 -> /dev/ttyACM21 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 27 -> /dev/ttyACM23 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 28 -> /dev/ttyACM9 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 29 -> /dev/ttyACM24 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 3 -> anon_inode:[eventfd]
lrwx------ 1 root root 64 Sep 24 01:44 30 -> /dev/ttyACM25 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 31 -> /dev/ttyACM26 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 32 -> /dev/ttyACM27 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 33 -> /dev/ttyACM28 (deleted)
lrwx------ 1 root root 64 Sep 24 04:35 34 -> /dev/ttyACM22 (deleted)
lrwx------ 1 root root 64 Sep 24 04:36 37 -> /dev/ttyACM30 (deleted)
lrwx------ 1 root root 64 Sep 24 04:49 38 -> /dev/ttyACM31 (deleted)
lrwx------ 1 root root 64 Sep 24 04:36 39 -> /dev/ttyACM29 (deleted)
lrwx------ 1 root root 64 Sep 24 01:44 4 -> anon_inode:[eventfd]
lrwx------ 1 root root 64 Sep 24 01:44 5 -> socket:[218298]
lrwx------ 1 root root 64 Sep 24 01:44 6 -> socket:[218304]
lrwx------ 1 root root 64 Sep 24 01:44 7 -> anon_inode:[eventfd]
lrwx------ 1 root root 64 Sep 24 01:44 8 -> socket:[218306]
lrwx------ 1 root root 64 Sep 24 01:44 9 -> /dev/ttyACM3 (deleted)

To me it look like a ModemManager bug: it must close descriptors that it 
no longer use at all.

This was tester with MM 1.4.0 from Debian Jessie with the generic driver.

Best Regards,
Jean-Christian