Re: [LAU] Simple, easy multithreaded circular buffer library for Linux?

On Fri, 2008-10-17 at 08:56 -0400, Paul Coccoli wrote:

> Why do you suspect you need memory barriers? My concern with
> 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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Received on Fri Oct 17 16:15:03 2008