On Sat, Oct 18, 2008 at 06:46:00PM +0200, Paul Davis wrote:
> I don't believe this logic is correct.
> ...
> The fact that in the revised version read_ptr is only read once changes
> nothing, since only thread ever modifies read_ptr. It doesn't matter how
> many times it accessed to do the computation - it will NEVER change its
> value during this computation because the computation happens in the
> reader thread and the reader thread is the only place where read_ptr is
> modified.
It doesn't matter how many times it is read, but it
does matter if it is ever written with a wrong value.
And this *did* happen in the old version at the end of
a read:
rb->read_ptr += n1;
rb->read_ptr &= rb->size_mask;
If a context switch occurs after the first statement,
and read_ptr > size (which can happen), then the writer
would believe there is more space to write than there
actually is.
Same at the other end: an incomplete update of write_ptr
(the addition is executed but not yet the mask) could make
the reader believe there is more to read than there actually
is.
Ciao,
-- FA Laboratorio di Acustica ed Elettroacustica Parma, Italia Lascia la spina, cogli la rosa. _______________________________________________ Linux-audio-user mailing list Linux-audio-user@email-addr-hidden http://lists.linuxaudio.org/mailman/listinfo/linux-audio-userReceived on Sun Oct 19 00:15:03 2008
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