Dtrace progress 20100725

Have been busy implementing the io::start and io::done probes. The mechanism
is different to the way Solaris/Apple do it - because we dont want
to annotate kernel (GPL) source code.

In getting this to work, I found a problem with the disassembler code.
The issue is handling a multi-byte NOP instruction (as emitted by GCC
to align jump targets to quad boundaries). Fortunately, Apple (or maybe Sun)
had already fixed the issue, so I grabbed an update to dis_tables.c.

In doing so, it exposed many more probe (fbt) points than previously seen.
I was previously seeing about 48,000 fbt probes on my Ubuntu 10.04 kernel.
After the above change, the number jumped to 250,000 - I assume because
now we more successfully find the multiple exit points in a function
rather than aborting when we hit one of these instructions.

Anyway, heres a simple example of the io::: provider working for
just one type of probe.

/home/fox/src/dtrace@vmub10-64: dtrace -l | grep -w io
    4         io                                               start
    5         io                                                done
    6         io                                                done
    7         io                                                done
    8         io                                                done
    9         io                                                done
   10         io                                                done
   11         io                                                done
   12         io                                                done
   13         io                                                done
   14         io                                                done
   15         io                                                done
/home/fox/src/dtrace@vmub10-64: dtrace -n io:::
dtrace: description 'io:::' matched 12 probes
CPU     ID                    FUNCTION:NAME
  0      6                            done:
  0     15                            done:
  0      6                            done:
  0     15                            done:
  0      6                            done:
  0      7                            done:
  0      6                            done:
  0      7                            done:
  0      6                            done:
  0      7                            done:
  0      6                            done:
  0      7                            done:
  0      6                            done:
  0      7                            done:
  ...

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