Re: PSA: If you run -current, beware!

On Thursday, February 05, 2015 10:48:54 AM John Baldwin wrote:
> On Thursday, February 05, 2015 04:22:23 PM Luigi Rizzo wrote:
> > On Thu, Feb 05, 2015 at 08:21:45AM -0500, John Baldwin wrote:
> > > On Thursday, February 05, 2015 08:48:33 AM Luigi Rizzo wrote:
> > ...
> > 
> > > > > > It is fixed (in the proper meaning of the word, not like worked
> > > > > > around,
> > > > > > covered by paper) by the patch at the end of the mail.
> > > > > > 
> > > > > > We already have a story trying to enable much less ambitious
> > > > > > option
> > > > > > -fno-strict-overflow, see r259045 and the revert in r259422.  I do
> > > > > > not
> > > > > > see other way than try one more time.  Too many places in kernel
> > > > > > depend on the correctly wrapping 2-complement arithmetic, among
> > > > > > others
> > > > > > are callweel and scheduler.
> > > > 
> > > > Rather than depending on a compiler option, wouldn't it be better/more
> > > > robust to change ticks to unsigned, which has specified wrapping
> > > > behavior?
> > > 
> > > Yes, but non-trivial.  It's also not limited to ticks.  Since the
> > > compiler
> > > knows when it would apply these optimizations, it would be nice if it
> > > could
> > > warn instead (GCC apparently has a warning, but clang does not).  Having
> > > people do a manual audit of every signed integer expression in the tree
> > > will take a long time.
> > 
> > I think I misunderstood the problem as being limited to ticks,
> > which is probably only one symptom of a fundamental change in behaviour
> > of the compiler.
> > Still, it might be worthwhile start looking at ints that ought to be
> > implemented as u_int
> 
> I actually agree, I just think we are stuck with -fwrapv in the interval,
> but it's probably not a short interval.  I think converting ticks to
> unsigned would be a good first start.

For the record, I agree.  However, I suspect that attempts to do so will have 
a non trivial number of bugs introduced.  We have a track record of recurring 
problems with tcp sequence number space arithmetic and tcp timing, partly 
because the wraparounds happens infrequently.

In the mean time, I feel that telling the compiler that it's OK to let it 
behave the way we expect (vs actively sabotaging it) is a viable stopgap.

-- 
Peter Wemm - peter_at_wemm.org; peter_at_FreeBSD.org; peter_at_yahoo-inc.com; KI6FJV
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