Re: [arm-gnu] Access to volatile bit-fields

[Kishore <kitts.mailinglists@xxxxxxxxx>]

On Thursday 02 Sep 2010 11:56:58 am you wrote:
> On Thu, 2010-09-02 at 11:10 +0530, Kishore wrote:
> > On Thursday 02 Sep 2010 10:24:29 am Alexander Zakharov wrote:
> > > Charles,
> > > 
> > > I do not want dispute or argue - by the end of the day its designer's
> > > choice, but I didn't spend this 15 years in the same company - I am
> > > freelance consultant and each shop has its own rules, tools, procedures
> > > and coding guidelines. It just happen that in current assignment
> > > bitfields are part of the design SOW since shop did and is using them
> > > heavily. The point though that I did try to make is that CS compiler is
> > > claimed to be ARM ABI compliant and as such any deviation from ARM ABI
> > > must be considered as a bug or flow (but not  a feature). By the end of
> > > the day if I as a coder do declare "volatile unsigned int: 5" I really
> > > mean it. Why compiler does choose access for me if I did directly
> > > instruct it to do unsigned int access respective to the architecture?
> > 
> > Now i am not entirely convinced about this. Lets look at your example
> > where you have a 5 bit variable (i) and lets say packed next to it,
> > you have another 5 bit variable (j) and they both sit in the same word
> > length...
> > 
> > Now.. in a multi threaded app, since writes to either i or j are not
> > atomic, if thread A did i = 2, it would first need to read the word
> > (which includes j), modify i and write back. Here any write to j
> > happening at the very same moment to j in thread B can corrupt it.
> > 
> > In other words, writes to bitfields are non atomic and can affect
> > adjacent variables and hence requires access protection around the
> > whole group of variables.
> 
> Shouldn't access protection be the responsibility of the programmer?  

Well. Yes. But in this case a "group" of bitfield variables need to access protected together. It may not very evident which one's belong to the group unless there was some well defined packing/behavior for bitfields.
-- 
Cheers!
Kishore