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SubjectRe: bad networking related lag in v2.6.22-rc2
Linus Torvalds wrote:
> There appear to be other obvious problems in the recent "cleanups" in this
> area..
> Look at
> psched_tdiff_bounded(psched_time_t tv1, psched_time_t tv2, psched_time_t bound)
> {
> return min(tv1 - tv2, bound);
> }
> and compare it to the previous code:
> #define PSCHED_TDIFF_SAFE(tv1, tv2, bound) \
> min_t(long long, (tv1) - (tv2), bound)
> and ponder how that "trivial cleanup" totally broke the thing.
> Hint: "psched_time_t" is an "u64". What does that mean for
> min(tv1 - tv2, bound);
> again, when "tv2" is larger than tv1. It _used_ to return a negative
> value. Now it returns a positive "bound" upper bound, because "tv1-tv2"
> will be used as a huge unsigned (and thus _positive_) integer. And was
> that accidental, or done on purpose?
> Sounds accidental to me, since you then want to return a "psched_tdiff_t",
> which is typedeffed to be "long".
> Doesn't sound very safe to me, especially since the commit message for
> this is "[NET_SCHED]: turn PSCHED_TDIFF_SAFE into inline function", and
> there's no indication that anybody realized that it changed semantics in
> the process.

I did realize it, but tv2 > tv1 can't happen and makes no sense for
the users of this function. I probably should have provided a more
detailed changelog entry.

> Hmm? What _should_ that thing do?

It is used to calculate the amount of tokens a tocken bucket has
accumulated since the last refill, thus we always have tv1 >= tv2
(modulo ktime wraps). In fact tv2 > tv1 was never properly
supported. This macro would have returned the negative long long
value, but all users assign it to a psched_tdiff_t (long), so
depending on the exact values, it might still be interpreted as a
large positive value. Additionally there was a second implementation
for the gettimeofday clocksource that didn't return the negative
difference but the bound value.

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