17 Oct
2008
17 Oct
'08
4:33 p.m.
On Fri, Oct 17, 2008 at 2:06 PM, Joern Nettingsmeier
<nettings(a)folkwang-hochschule.de> 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...