On Fri, Oct 17, 2008 at 2:06 PM, Joern Nettingsmeier <nettings@xxxxxxxxxxxxxxxxxxxxxx> wrote: > excuse my chiming in here, i'm not really much of a c programmer... but > how can this > - rb->read_ptr += n1; > - rb->read_ptr &= rb->size_mask; > + rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask; > fix anything? > > iiuc, both versions are equivalent. a context switch could happen just > as well after the parenthesis has been computed..!? > putting stuff on one line doesn't make it atomic. maybe you are now > getting another compiler optimization that helps to hide the bug? > They are not equivalent. The first version modifies rb->readptr twice; the second version modifies once. I can't say with any certainty that that fixes the problem, but it makes me feel better (and happens to pass this test). There's no chance that a compiler optimization is hiding the bug, because we haven't turned on optimization. HOWEVER, if you go back to the original version and REMOVE the volatile qualifiers, then re-compile with -O2, NOW a compiler optimization is hiding the bug (and the test passes)! The rb->read_ptr is presumably cached in a register and hence only modified once (giving the same affect as the patch). Now THAT's interesting... _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
