Pls sanity check my semtimedop(2) implementation

[John Baldwin]

On Thursday 17 July 2008 01:42:31 pm Michael B Allen wrote:
> On Thu, Jul 17, 2008 at 10:05 AM, John Baldwin <jhb at freebsd.org> wrote:
> > On Saturday 12 July 2008 07:11:26 pm Michael B Allen wrote:
> >> Hi,
> >>
> >> Below is a semtimedop(2) implementation that I'm using for FreeBSD. I
> >> was hoping someone could look it over and tell me if they think the
> >> implementation is sound.
> >>
> >> The code seems to work ok but when stressing the FreeBSD build of my app
> >> I have managed to provoke errors related to concurrency (usually when a
> >> SIGALRM goes off). The Linux build works flawlessesly so I'm wondering
> >> about this one critical function that is different.
> >>
> >> Do you think it would make any difference if I used
> >> ITIMER_VIRTUAL / SIGVTALRM instead of ITIMER_REAL / SIGALRM?
> >>
> >> Or perhaps I should be using a different implementation entirely?
> >
> > What specific races are you seeing?  The timer is firing too early, too
> > late?
>
> It's very difficult to tell. I can only trigger the issue very
> occasionally running my torture test such that any diagnostic logging
> changes the results.
>
> And at this point I'm not sure my semtimedop implementation is
> responsible. I have not seen the issue since fixing the race pointed
> out by Mikko (although I have not tried very hard to provoke it).
>
> For now, I'm satisfied since I do not think the issue will be
> triggered in the wild. I hate to use signals for anything but as much
> as I try, there's just no other way to implement semtimedop within a
> single largely self-contained function. In the future I will likely
> use another process in the application that uses select(2) as an
> "event service" to post on semaphores after a certain time period.
> Unfortunately, right now, that service ultimately calls semtimedop so
> I'll save it for a rainy day.
>
> Although if you implemented semtimedop(2) into the FreeBSD API that
> would work too :-)

POSIX semaphores (sem_open(3), sem_create(3), etc.) do have a 
sem_timedwait(3).  However, POSIX semaphores have several bugs in 6.x and 7.x 
(they should work a lot better in 8).  If you want I can give you a patch for 
6.x or 7.x that backports the 8.x POSIX semaphores.

-- 
John Baldwin