swait: strengthen language to discourage use

We already earlier discouraged people from using this interface in
commit 88796e7e5c ("sched/swait: Document it clearly that the swait
facilities are special and shouldn't be used"), but I just got a pull
request with a new broken user.

So make the comment *really* clear.

The swait interfaces are bad, and should not be used unless you have
some *very* strong reasons that include tons of hard performance numbers
on just why you want to use them, and you show that you actually
understand that they aren't at all like the normal wait/wakeup
interfaces.

So far, every single user has been suspect.  The main user is KVM, which
is completely pointless (there is only ever one waiter, which avoids the
interface subtleties, but also means that having a queue instead of a
pointer is counter-productive and certainly not an "optimization").

So make the comments much stronger.

Not that anybody likely reads them anyway, but there's always some
slight hope that it will cause somebody to think twice.

I'd like to remove this interface entirely, but there is the theoretical
possibility that it's actually the right thing to use in some situation,
most likely some deep RT use.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swait.h

@@ -8,7 +8,19 @@
 #include <asm/current.h>
 
 /*
- * Simple wait queues
+ * BROKEN wait-queues.
+ *
+ * These "simple" wait-queues are broken garbage, and should never be
+ * used. The comments below claim that they are "similar" to regular
+ * wait-queues, but the semantics are actually completely different, and
+ * every single user we have ever had has been buggy (or pointless).
+ *
+ * A "swake_up()" only wakes up _one_ waiter, which is not at all what
+ * "wake_up()" does, and has led to problems. In other cases, it has
+ * been fine, because there's only ever one waiter (kvm), but in that
+ * case gthe whole "simple" wait-queue is just pointless to begin with,
+ * since there is no "queue". Use "wake_up_process()" with a direct
+ * pointer instead.
  *
  * While these are very similar to regular wait queues (wait.h) the most
  * important difference is that the simple waitqueue allows for deterministic