linuxaudio.org

On Tue, Jul 12, 2011 at 4:20 PM, Olivier Guilyardi  wrote:
> Quite interestingly, I have noticed that discussions about memory barriers are

quoted text
> often somehow endless. What happened in the past is that I saw countless

> discussions about whether they are needed, whether they are not, and people

> would argue a lot and passionately.

I think the problem is that memory barriers are almost never required

when writing "normal" code, and so people (including myself) are not

exposed to their implementation or their use very much. and indeed,

there are precious few library implementations of memory barriers, nor

are they widely documented in a way that suggests that their use is

"normal".
by contrast, they get used all over the place in the kernel

(relatively speaking), so if you tend to have a lot of exposure to

kernel code, then calls to mb() or whatever they use these days will

be quite familiar.
there's the additional problem that this discussion normally ends up

confusing two separate topics that many people seem to think are the

same (they are not):
   1) do you need to actively ensure correct thread-level synchronization

          between the reader and writer of a single-reader/single-writer FIFO?

	  Put differently, do you need to use synchronization mechanisms

          semantically equivalent to a mutex to ensure that any

arbitrary execution

          order of the 2 threads does not cause incorrect behaviour?
   2) do you need memory barriers to ensure correct synchronization

         for this kind of data structure in the face of possible hardware level

         instruction reordering?
My feeling is that the answer to (1) is "no" and the answer to (2) is "yes".

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