Re: [Valgrind-users] Marking blocks as write-once/grow-only

[Alex B. <ker...@be...>]

Julian Seward <js...@ac...> writes:

>> Is it possible for the DRD/Helgrind tools to detect this sort of
>> double-write access behaviour?
>
> Both of them should be able to detect a write-vs-write race, if
> that's what you mean.

But is that only if two threads race to write the same location at the
same time?

>
>> Could I instrument QEMU so it marked the
>> codegen buffer as one that should only grow upwards (modulo-patchable
>> bits) so if anything re-wrote the buffer it could trigger an error?
>
> This is confusing.  Both tools are able to detect races at a byte level
> granularity.  If you can show that QEMU doesn't race on individual writes
> to its code buffer, isn't that good enough from a correctness
> perspective?

What I think has happened is:

  * thread a writes from code_buf_start a series of operations
  * thread b writes from code_buf_start over the top of thread a
  * thread b finalises the code buffer at code_buf_end<b>
  * thread a finalises the code buffer at code_buf_end<a>

The result is a corrupted code buffer where one threads output has been
stomped on by the other. I don't think they raced on the individual
writes, just over wrote the previous work on the next schedule.
>
> J

-- 
Alex Bennée