Presently, s_lock.h insists that every platform provide TAS(), and
its header comment doesn't say which of the macros a new platform
actually needs to supply. The real contract is that a platform
must provide either S_LOCK() or a TAS() for the default S_LOCK() to
be built on.
To better express that, this commit moves the "no spinlock support"
error to where the default S_LOCK() is defined, compiles s_lock()
only when the default S_LOCK() is in use, and provides a default
TAS_SPIN() only when there is a TAS() to base it on. The direct
callers of s_lock() in the spinlock tests are adjusted to match.
The header comment is reworked accordingly, dropping the note about
pre-9.5 volatile requirements and the caution about TAS()
spuriously failing along the way.
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/afkUeI7UhacZ5ZFm%40nathan
d9b5a63f49