MFC r344855 (by jhb):

Drop "All rights reserved" from my copyright statements.

Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D19485

MFC r312890,r313386,r313390:

Sprinkle __read_mostly on backoff and lock profiling code.

==

locks: change backoff to exponential

Previous implementation would use a random factor to spread readers and
reduce chances of starvation. This visibly reduces effectiveness of the
mechanism.

Switch to the more traditional exponential variant. Try to limit starvation
by imposing an upper limit of spins after which spinning is half of what
other threads get. Note the mechanism is turned off by default.

==

locks: follow up r313386

Unfinished diff was committed by accident. The loop in lock_delay
was changed to decrement, but the loop iterator was still incrementing.

MFC r303562,303563,r303584,r303643,r303652,r303655,r303707:

rwlock: s/READER/WRITER/ in wlock lockstat annotation

==

sx: increment spin_cnt before cpu_spinwait in xlock

The change is a no-op only done for consistency with the rest of the file.

==

locks: change sleep_cnt and spin_cnt types to u_int

Both variables are uint64_t, but they only count spins or sleeps.
All reasonable values which we can get here comfortably hit in 32-bit range.

==

Implement trivial backoff for locking primitives.

All current spinning loops retry an atomic op the first chance they get,
which leads to performance degradation under load.

One classic solution to the problem consists of delaying the test to an
extent. This implementation has a trivial linear increment and a random
factor for each attempt.

For simplicity, this first thouch implementation only modifies spinning
loops where the lock owner is running. spin mutexes and thread lock were
not modified.

Current parameters are autotuned on boot based on mp_cpus.

Autotune factors are very conservative and are subject to change later.

==

locks: fix up ifdef guards introduced in r303643

Both sx and rwlocks had copy-pasted ADAPTIVE_MUTEXES instead of the correct
define.

==

locks: fix compilation for KDTRACE_HOOKS && !ADAPTIVE_* case

==

locks: fix sx compilation on mips after r303643

The kernel.h header is required for the SYSINIT macro, which apparently
was present on amd64 by accident.

Approved by: re (gjb)