Re: rwlock(9) and mutex(9) definitions

On Tue, Oct 26, 2021 at 06:52:21PM -0700, Gleb Smirnoff wrote:
>   Hi,
> 
> [To: list constructed with help of git blame]
> 
> despite manual pages describe locking functions as voids in
> reality some of them (not all) are preprocessor defines wrapped
> in "do {} while (0)".
> 
> Such wraps don't really behave as a true void. For example you
> can not tail call them:
> 
> void
> smartass_lock(lock, clue)
> {
> 	if (clue)
> 		return (rw_rlock(lock));
> 	else
> 		return (rw_wlock(lock));
> }
> 
> This will fail on rw_wlock, but not on rw_rlock. However, if you
> have WITNESS it will compile correctly :)
> 
> So, we need either make these function "static inline void" in
> mtx.h and rwlock.h, or wrap them in __extension__ ({ }). Btw, the
> latter is already done for mtx_trylock_spin() by kib in 90b581f2cc327.
> Of course for try-lock functions inability of tail call is a bigger
> issue then for voids. However, voids should be fixed as well, I believe.
> 
> Your call? "static inline" or "__extension__ ({ })"?

Usually defines are easier.  For static inline functions, compiler parses
and validates the definition regardless of its use.  So for instance if
the header is included into userspace compilation consumers, static inlines
break.

If not that, providing real parseable definitions are better on all other
counters.