Fix for ngx_unlock() and thus race conditions in AIO mechanism

[Mindaugas Rasiukevicius]

Maxim Dounin <mdounin at mdounin.ru> wrote:
> Patch that follows the same logic as used in ngx_update_time(), that is,
> with an explicit ngx_memory_barrier() call before ngx_unlock(), and no
> barrier semantics in ngx_unlock() itself:

I can just point out that pretty much all spin-lock implementations
provide memory ordering guarantees; this is 1990s discussion which has
been settled by now with a consensus that certain ordering guarantees
should be provided by the synchronisation primitives.  If you will look
into the kernels (be it Linux, BSD or Solaris -- it was Sun who de facto
set the standard back in the day), libraries (say libpthread) or other
projects, (e.g. PostgreSQL) -- you will find the right barrier in the
unlock operation.

If you do not provide these guarantees, then you provide something what
is different from a textbook implementation of a spin-lock.  It is rather
dangerous and unlikely to prevent from bugs.

-- 
Mindaugas