Enable DAHDI to detect if an operation on a file handle refers to a
channel that may have been unregistered. This can occur, for example,
when a board driver is hot-swapped out in a live system.
This patch ensures that file->private_data is always properly set for
any open channel, and it's set back to NULL when a channel is
unregistered. This way file->private_data can be used to check whether
it's valid to perform an operation on the channel. (NOTE: There is
still a race condition here if the driver was unbound on one processor
during the window of time between when file->private_data was checked
and the system call finishes).
Also, since DAHDI should only return -ENODEV on read or write when there
was a surprise device removal on a running system this sleep can prevent
the system from becoming unresponsive if the userspace application does
not check for the -ENODEV error and constantly tries to call read with
elevated privileges.
(issue #17669)
Reported by: tzafrir
Tested by: sruffell
Review: https://reviewboard.asterisk.org/r/905/
Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Origin: http://svnview.digium.com/svn/dahdi?view=rev&rev=9353
git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/branches/2.4@9662 a0bf4364-ded3-4de4-8d8a-66a801d63aff
43ebb99002