cvs commit: src/sys/kern tty_pty.c

[Bruce Evans]

On Fri, 29 Sep 2006, Martin Blapp wrote:

>> I think I found the bug while looking for problems near vgonel().  We're
>> nowhere near ready to free devices in in last-close, since vgonel() doesn't
>> do anything to evict processes from device functions before it forces the
>> device closed.  Drivers must be aware of the problem.  The tty driver
>> already is.  See the comments about t_gen near tty_close() and ttysleep().
>> t_gen must live across close so that any processes in device functions can
>> check it after they wake up, and the tty and device data structures must
>> live across last- close to hold t_gen and anything else needed for the
>> device functions to return.  Sleeping device functions normally wake up
>> after last-close returns.
>
> So what can we do to destroy devices after last close ? Mark them with some
> flag and handle this during device operations ?

Destroy them after a timeout.  More accurate counts of device use would
still be needed to ensure that the device is not in use when the timeout
fires, but it would be fairly safe to just assume that the device is
not in use if it has been closed for some time.  Note that the device
may still be in use when close completes even for normal closes.  This
can happen when the close blocks.  When close completes, it can't
destroy things since it doesn't really know what is in use.  However,
if the device is still in use, if it is open then it won't be destroyed
later (a new open would have reset the timeout), and if it is not open
then any activity on the device should complete soon after the close
completes since one of close's functions (especially for revoke()) is
unblock other activity on the device by flushing any i/o that it gave
up on.

Bruce