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