Re: [Valgrind-users] Helgrind doesn't handle bit field correctly

[David C. <dcc...@ac...>]

On 3/22/2013 4:52 PM, you wrote:
>
> When threads are writing to bit fields, there are mutex locks. It's an 
> existing condition and it worked well from many years. So I wonder if 
> there is a way to get around the "false" errors from Helgrind? Thanks.
>

Without seeing the precise code that is creating the locks and accessing 
the variables, it is hard to be certain that it is in fact safe.  Can 
you reduce it to a minimal sample?  Are you using only POSIX pthread 
primitives, or properly describing the lock primitives you are using?  
If you can create a small sample with no proprietary code that exhibits 
the problems you are seeing, someone with more experience using Helgrind 
(or one of the tool's authors) can comment on it.

One thing that may be causing problems is that Valgrind and its analysis 
tools (such as Helgrind) are working with assembly language, not a 
high-level language such as C.  Thus it does not know the intent of your 
code - that there are two independent sets of bit fields in the record, 
and that threads will never (or at least should never) modify the 
"wrong" bit fields.  It only sees that a data word was written by one 
thread, but another thread still has an old value.  The question then 
becomes "why does Helgrind think your code has an old value somewhere?"

-- 
     David Chapman      dcc...@ac...
     Chapman Consulting -- San Jose, CA
     Software Development Done Right.
     www.chapman-consulting-sj.com