linuxaudio.org

Fons Adriaensen wrote:

quoted text
> 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

>> 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.

I don't see why it's fragile. There's no reason for the compiler to modify

rb->read_ptr if not asked to. Buf if that's the case, because of some sort of

dark voodoo optimization, then we could use a temporary variable to hold the

intermediary result. Better, all computation could be done on this temporary

variable and transfered into rb->read_ptr once finished, so that modifying the

later only involves memory copy.
> One real solution is to *not* use the size_mask

quoted text
> 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.

You class looks good, but some work is needed to provide a Jack backward

compatible API based on it.
IMO, one important thing is to include unit/regression tests, for the ringbuffer

as well as general Jack operation, in the official jack distribution, that one

can run with a simple "make test". I'll try to do this for this test suite I

wrote, once cleaned up.
--

  Olivier Guilyardi / Samalyse

_______________________________________________

Linux-audio-user mailing list

Linux-audio-user@lists.linuxaudio.org

http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user