Instead of embedding a lock bit in rtree leaf elements, we associate extents
with a small set of mutexes. This gets us two things:
- We can use the system mutexes. This (hypothetically) protects us from
priority inversion, and lets us stop doing a backoff/sleep loop, instead
opting for precise wakeups from the mutex.
- Cuts down on the number of mutex acquisitions we have to do (from 4 in the
worst case to two).
We end up simplifying most of the rtree code (which no longer has to deal with
locking or concurrency at all), at the cost of additional complexity in the
extent code: since the mutex protecting the rtree leaf elements is determined by
reading the extent out of those elements, the initial read is racy, so that we
may acquire an out of date mutex. We re-check the extent in the leaf after
acquiring the mutex to protect us from this race.
This lets us specify whether and how mutexes of the same rank are allowed to be
acquired. Currently, we only allow two polices (only a single mutex at a given
rank at a time, and mutexes acquired in ascending order), but we can plausibly
allow more (e.g. the "release uncontended mutexes before blocking").
We don't touch witness at all when config_debug == false. Let's only pay the
memory cost in malloc_mutex_s when needed. Note that when !config_debug, we keep
the field in a union so that we don't have to do #ifdefs in multiple places.
This is part of a broader change to make header files better represent the
dependencies between one another (see
https://github.com/jemalloc/jemalloc/issues/533). It breaks up component headers
into smaller parts that can be made to have a simpler dependency graph.
For the autogenerated headers (smoothstep.h and size_classes.h), no splitting
was necessary, so I didn't add support to emit multiple headers.
