linuxaudio.org

On Fri, Oct 17, 2008 at 08:06:55PM +0200, Joern Nettingsmeier wrote:
> excuse my chiming in here, i'm not really much of a c programmer... but

quoted text
> how can this

>   -  rb->read_ptr += n1;

>   -  rb->read_ptr &= rb->size_mask;

>   +  rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask;

> fix anything?

>

> iiuc, both versions are equivalent. a context switch could happen just

> as well after the parenthesis has been computed..!?

> putting stuff on one line doesn't make it atomic. maybe you are now

> getting another compiler optimization that helps to hide the bug?

The idea is that it is very unlikely that the compiler

would store the intermediate result in rb->read_ptr,

and so this value is updated only once.
I agree it is fragile, there is nothing stopping

the compiler from doing it wrong. Unless the

the pointers are made volatile again.
One real solution is to *not* use the size_mask

at this point, but apply it only when read_ptr

and write_ptr are read. So only a single addition

remains. The pointers are allowed to grow, they

will wrap around at the word size, but since the

buffer size is a power of two they are still

correct when that happens.
This is what is done in the C++ class I

posted earlier.
Ciao,
--

FA
Laboratorio di Acustica ed Elettroacustica

Parma, Italia
Lascia la spina, cogli la rosa.
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