[PATCH v6 0/9] latched RB-trees and __module_address()

[Peter Zijlstra <peterz@infradead.org>]

This series is aimed at making __module_address() go fast(er).

The reason for doing so is that most stack unwinders use kernel_text_address()
to validate each frame. Perf and ftrace (can) end up doing a lot of stack
traces from performance sensitive code.

On the way there it:
 - annotates and sanitizes module locking
 - introduces the latched RB-tree
 - employs it to make __module_address() go fast.

I've build and boot tested this on x86_64 with modules and lockdep
enabled.  Performance numbers (below) are done with lockdep disabled.

As previously mentioned; the reason for writing the latched RB-tree as generic
code is mostly for clarity/documentation purposes; as there are a number of
separate and non trivial bits to the complete solution.

As measured on my ivb-ep system with 84 modules loaded; the test module reports
(cache hot, performance cpufreq):

          avg +- stdev
Before:   611 +- 10 [ns] per __module_address() call
After:     17 +-  5 [ns] per __module_address() call

PMI measurements for a cpu running loops in a module (also [ns]):

Before:	Mean: 2719 +- 1, Stdev: 214, Samples: 40036
After:  Mean:  947 +- 0, Stdev: 132, Samples: 40037

Note; I have also tested things like: perf record -a -g modprobe
mod_test, to make 'sure' to hit some of the more interesting paths.

Changes since last time:

 - rebased against Rusty's tree
 - raw_read_seqcount_latch() -- (mingo)

Based on rusty/linux.git/pending-rebases; please consider for 4.2

Thanks!