[TUHS] Another odd comment in V6

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Feb 15 00:14:24 AEST 2017


    > From: Paul Ruizendaal

    > There's an odd comment in V6, in tty.c, just above ttread():
    > ...
    > That comment is strange, because it does not describe what the code
    > does.

I can't actually find anyplace where the PC is backed up (except on a
segmentation fault, when extending the stack)?

So I suspect that the comment is a tombstone; it refers to what the code did
at one point, but no longer does.

    > The comment isn't there in V5 or V7.

Which is consistent with it documenting a temporary state of affairs...


    > I wonder if there is a link to the famous Gabriel paper

I suspect so. Perhaps they tried backing up the PC (in the case where a system
call is interrupted by a software interrupt in the user's process), and
decided it was too much work to do it 'right' in all instances, and punted.

The whole question of how to handle software interrupts while a process is
waiting on some event, while in the kernel, is non-trivial, especially in
systems which use the now-universal approach of i) writing in a higher-level
stack oriented language, and ii) 'suspending' with a sub-routine call chain on
the kernel stack.

Unix (at least, in V6 - I'm not familiar with the others) just trashes the
whole call stack (via the qsav thing), and uses the intflg mechanism to notify
the user that a system call was aborted. But on systems with e.g. locks, it
can get pretty complicated (try Googling Multics crawl-out). Many PhD theses
have looked at these issues...


    > Actually, research Unix does save the complete state of a process and
    > could back up the PC. The reason that it doesn't work is in the syscall
    > API design, using registers to pass values etc. If all values were
    > passed on the stack it would work.

Sorry, I don't follow this?

The problem with 'backing up the PC' is that you 'sort of' have to restore the
arguments to the state they were in at the time the system call was first
made. This is actually easier if the arguments are in registers.

I said 'sort of' because the hard issue is that there are system calls (like
terminal I/O) where the system call is potentially already partially executed
(e.g.  a read asking for 10 characters from the user's console may have
already gotten 5, and stored them in the user's buffer), so you can't just
simply completely 'back out' the call (i.e. restore the arguments to what they
were, and expect the system call to execute 'correctly' if retried - in the
example, those 5 characters would be lost).

Instead, you have to modify the arguments so that the re-tried call takes up
where it left off - in the example above, tries to read 5 characters, starting
5 bytes into the buffer). The hard part is that the return value (of the
number of characters actually read) has to count the 5 already read! Without
the proper design of the system call interface, this can be hard - how does
the system distinguish between the _first_ attempt at a system call (in which
the 'already done' count is 0), and a _later_ attempt? If the user passes in
the 'already done' count, it's pretty straightforward - otherwise, not so
much!

Alan Bawden wrote a good paper about PCLSR'ing which explores some of these
issues.

	Noel


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