[PATCH] spin loop primitives for busy waiting

[David Laight]

From: Linus Torvalds
> Sent: 11 May 2017 19:48
...
> The one question I have is about "spin_on_cond()": since you
> explicitly document that the "no spinning" case is expected to be the
> default, I really think that the default implementation should be
> along the lines if
> 
>   #define spin_on_cond(cond) do { \
>     if (unlikely(!(cond))) { \
>         spin_begin(); do spin_cpu_relax(); while (!(cond)); spin_end(); \
>     } \
>   } while (0)
> 
> which will actually result in better code generation even if
> spin_begin/end() are no-ops, and just generally optimizes for the
> right behavior (ie do the spinning out-of-line, since by definition it
> cannot be performance-critical after the first iteration).

At least some versions of gcc convert while (cond) do {body}
into if (cond) do {body} while (cond) even when 'cond'
is a non-trivial expression and 'body' is trivial.
The code-bloat is silly.
No point enforcing the 'optimisation' here.

	David