[Bug 246597] Race condition in mountd

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246597

Rick Macklem <rmacklem at FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|bugs at FreeBSD.org            |rmacklem at FreeBSD.org
             Status|New                         |Open

--- Comment #2 from Rick Macklem <rmacklem at FreeBSD.org> ---
Well, handling of SIGHUP to reload the exports file is asynchronous,
so I don't think the race Patryk is referring to is affected by handling
the next request before reloading the exports file.

I think he is referring to a case where:
- exports file is updated, sighup posted
- mountd is in get_exportslist();
  --> while mountd is in getexport_list()…
      - exports file is updated and sighup is posted
- mountd completes get_exportlist(), but does not
  do so again, since it set got_sighup = 0 after
  the last sighup posted was handled.

Patryk, do I have the above correct?

Now, your fix will fix this problem, but it introduces another one.
- Some file servers (Peter Eriksson) have 80000+ file systems exported
  on them. Yes, I did type the correct number above. He creates a
  separate file system for each student, etc. (Most of his servers have
  20000+, but one is over 80000.)
- When exports are (re)loaded on his servers, it can take minutes.
  I think r348590 reduced the time from something like 20minutes to
  5minutes, but it is still minutes.
--> What happens if a sighup is posted to mountd once every couple
    of minutes with your patch? All mountd does is reload exports
    over and over and over again (lyrics of an old Beatles toon?).
    The current code can suffer from this too, but it ignores sighups
    posted while it is in get_exportlist() which at least partially
    avoids this.
    - Part of the problem is that FreeBSD's mount command posts a
      sighup to mountd every time a mount is done, just in case that
      new file system needs to be exported. (/etc/exports hasn't
      changed, but it does not know if this new file system needs
      to be exported. I've never liked this, but it is a long
      standing FreeBSD tradition, so I dare not change it.;-)

So, what can be done to make sure the updated exports file gets
processed without just doing get_exportlist() over and over..?

I think I'll post somewhere like FreeBSD-net@, since it is somewhat
like a networking problem where a delay avoids congestion.

An example solution might be:
- time how long get_exportlist() takes.
- note that sighup(s) have been posted.
- do get_exportlist() again, but only after a delay at least as large
  as the time the last get_exportlist() took.
--> That way, the updated exports would be processed, but mountd
    would still do other RPC work as well.
--> It might be that even a delay of 1sec between get_exportlist() calls
    would be sufficient to allow it to do the Mount RPC requests.
    (There shouldn't be that many of them.)

I've actually known about this for quite a while, but since I didn't
have a good solution, I left it as is.

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