linuxaudio.org

On Fri, Oct 17, 2008 at 2:06 PM, Joern Nettingsmeier

 wrote:

quoted text
> 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...

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