Re: [PATCH] Profile likely/unlikely macros

[Nick Piggin]

Jörn Engel wrote:

> Admitted, I'm a bit slow at times.  But why does this matter?
> According to my fairly limited brain, you take a potentially expensive
> barrier, so you pay with a bit of runtime.  What you buy is a smaller
> critical section, so you can save some runtime on other cpus.  When
> optimizing for the common case, which is one cpu, this is a net loss.
>
> There must be some correctness issue hidden that I cannot see.  Can
> you explain that to me?

Another CPU may find the bit clear, enter the critical section,
and load the old `likeliness_head' (value before being changed
by this CPU).

Then it stores the old value to likeliness->next, and overwrites
likeliness_head.

One CPU's update has now gotten lost.

(there are probably other examples, like missing likliness->type)

--
SUSE Labs, Novell Inc.
Send instant messages to your online friends http://au.messenger.yahoo.com 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/