On 12/04/2015 05:05 PM, Linus Torvalds wrote:
On Fri, Dec 4, 2015 at 12:51 PM, Waiman Long<waiman.l...@hpe.com> wrote:
Will the following work?
Are we trying to win some obfuscated C contest here?
Just make it do something like (skipping backslashes to make it easier
to type and read)
#define smp_cond_load_acquire(ptr, cond_expr) ({
typeof(*ptr) VAL;
for (;;) {
VAL = READ_ONCE(*ptr);
if (cond_expr) break;
cpu_relax();
}
smp_rmb();
VAL;
})
and then you'd have it be
val = smp_cond_load_acquire(&lock->val.counter,
!(VAL& _Q_LOCKED_PENDING_MASK));
which is at least halfway legible. Not some odd "fragments of
expressions" interfaces unless absolutely required, please.
It is just some random thought that I have. I am not saying that it is
the right way to go.
Of course, I suspect we should not use READ_ONCE(), but some
architecture-overridable version that just defaults to READ_ONCE().
Same goes for that "smp_rmb()". Because maybe some architectures will
just prefer an explicit acquire, and I suspect we do *not* want
architectures having to recreate and override that crazy loop.
How much does this all actually end up mattering, btw?
Linus
I think what Will want to do is to provide an architecture specific
replacement for the whole macro, not just part of it. So using READ_ONCE
should be fine.
Cheers,
Longman
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