linuxaudio.org

On Sat, Oct 18, 2008 at 06:46:00PM +0200, Paul Davis wrote:
> I don't believe this logic is correct.

quoted text
> ...

> The fact that in the revised version read_ptr is only read once changes

> nothing, since only thread ever modifies read_ptr. It doesn't matter how

> many times it accessed to do the computation - it will NEVER change its

> value during this computation because the computation happens in the

> reader thread and the reader thread is the only place where read_ptr is

> modified. 

It doesn't matter how many times it is read, but it

does matter if it is ever written with a wrong value.

And this *did* happen in the old version at the end of

a read:
   rb->read_ptr += n1;

   rb->read_ptr &= rb->size_mask;
If a context switch occurs after the first statement,

and read_ptr > size (which can happen), then the writer

would believe there is more space to write than there

actually is.

Same at the other end: an incomplete update of write_ptr

(the addition is executed but not yet the mask) could make

the reader believe there is more to read than there actually

is.
Ciao,
--

FA
Laboratorio di Acustica ed Elettroacustica

Parma, Italia
Lascia la spina, cogli la rosa.
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