lkml.org 
[lkml]   [2007]   [May]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-05-23 19:23    [from the cache]
©2003-2011 Jasper Spaans. Advertise on this site