Herbert Xu wrote:
The original reason for the return value is so you can do
if (WARN_ON(impossible_condition)) {
attempt_to_continue;
}
instead of
if (unlikely(impossible_condition)) {
WARN_ON(1);
attempt_to_continue;
}
(Hm, WARN_ON(1) is pretty ugly; we should probably have a WARN() as well.)
Why is the second one any better than the first? It's a line less code,
but that doesn't seem like a big deal. It's not like passing the actual
condition into WARN_ON is useful, because it doesn't try to print it
out. And "if (WARN_ON_ONCE(cond)) ..." is arguably more useful (since it
encapsulates the printing once logic), but also very unclear (does it
evaluate true once or every time?).
There are certainly lots of places in the kernel which could use
if(WARN_ON(...)), but I haven't found any places which actually do. I
just don't see what benefit you would gain in converting things to using
if(WARN_ON(...)) anyway.
Oh and yes the unlikely does make a difference in a statement
expression.
I was thinking something like
unlikely(({
...
}))
is a bit more obvious in terms of imagining how it would get expanded
and evaluated.
J
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