Re: [PATCH v2] perfcounters: record time running and time enabledfor each counter

From: Andrew Morton
Date: Sun Mar 22 2009 - 05:03:32 EST


On Sun, 22 Mar 2009 10:13:35 +1100 Paul Mackerras <paulus@xxxxxxxxx> wrote:

> Andrew Morton writes:
>
> > Perhaps one of the reasons why this code is confusing is the blurring
> > between the "time" at which an event occured and the "time" between the
> > occurrence of two events. A weakness in English, I guess. Using the term
> > "interval" in the latter case will help a lot.
>
> Except that we aren't measuring an "interval", we're measuring the
> combined length of a whole series of intervals. What's a good word
> for that?

foo_total_time?

It doesn't matter so much if the thing has a comment at the definition site.

> > > + atomic64_t child_time_enabled;
> > > + atomic64_t child_time_running;
> >
> > These read like booleans, but why are they atomic64_t's?
>
> OK so this file could use more comments, but I did answer that
> question in the patch description.
>
> > > - return put_user(cntval, (u64 __user *) buf) ? -EFAULT : sizeof(cntval);
> > > + if (count != n * sizeof(u64))
> > > + return -EINVAL;
> > > +
> > > + if (!access_ok(VERIFY_WRITE, buf, count))
> > > + return -EFAULT;
> > > +
> >
> > <panics>
> >
> > Oh.
> >
> > It would be a lot more reassuring to verify `uptr', rather than `buf' here.

This?

> > The patch adds new trailing whitespace. checkpatch helps.
> >
> > > + for (i = 0; i < n; ++i)
> > > + if (__put_user(values[i], uptr + i))
> > > + return -EFAULT;
> >
> > And here we iterate across `n', whereas we verified `count'.
>
> And the fact that we just verified count == n * 8, four lines above,
> doesn't give you any comfort?

access_ok(..., uptr, n * sizeof(*uptr))

might be most robust.

Or fix up the types (if needed) and copy the whole thing with copy_to_user()

Is it really so performance-sensitive that we can't use plain old put_user()?
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