linuxaudio.org

On Fri, 2008-10-17 at 08:56 -0400, Paul Coccoli wrote:
> Why do you suspect you need memory barriers?  My concern with

quoted text
> ringbuffer.c is the non-atomic ops on the read and write pointers.

> They're marked volatile, but what I think you really want is make all

> ops on those fields atomic.  Stuff like this:

>

> rb->read_ptr += n1;

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

>

> Looks like a problem to me.  What happens if there's a context switch

> in between those 2 statements?

this is the core of the "idea" of the thread-safe ringbuffer.
step back. answer the question. what *does* happen?
it means that the read_ptr has an unnecessarily high value, which means

that we *under*-estimate the space available for writing.. ditto for the

write_ptr case, where we *under*-estimate the data available for

reading. 
we can and *do* anticipate that context switches will occur there -

there is no general way to that execute that operation

(increment-and-mask) in a lock-free atomic way. the idea is that even if

it does happen, the resulting error prevents the buffer from being

misused. of course, its a little inefficient. but until we get cpu's

with atomic inc-and-mask instructions, its the price we pay for going

lock-free (and its a relatively small cost).
--p
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