The implementation is hybrid active/passive spin/blocking mutex.
The design minimizes amount of context switches and futex calls.
The idea is that all critical sections in runtime are intentially
small, so pure blocking mutex behaves badly causing
a lot of context switches, thread parking/unparking and kernel calls.
Note that some synthetic benchmarks become somewhat slower,
that's due to increased contention on other data structures,
it should not affect programs that do any real work.

On 2 x Intel E5620, 8 HT cores, 2.4GHz
benchmark                     old ns/op    new ns/op    delta
BenchmarkSelectContended         521.00       503.00   -3.45%
BenchmarkSelectContended-2       661.00       320.00  -51.59%
BenchmarkSelectContended-4      1139.00       629.00  -44.78%
BenchmarkSelectContended-8      2870.00       878.00  -69.41%
BenchmarkSelectContended-16     5276.00       818.00  -84.50%
BenchmarkChanContended           112.00       103.00   -8.04%
BenchmarkChanContended-2         631.00       174.00  -72.42%
BenchmarkChanContended-4         682.00       272.00  -60.12%
BenchmarkChanContended-8        1601.00       520.00  -67.52%
BenchmarkChanContended-16       3100.00       372.00  -88.00%
BenchmarkChanSync                253.00       239.00   -5.53%
BenchmarkChanSync-2             5030.00      4648.00   -7.59%
BenchmarkChanSync-4             4826.00      4694.00   -2.74%
BenchmarkChanSync-8             4778.00      4713.00   -1.36%
BenchmarkChanSync-16            5289.00      4710.00  -10.95%
BenchmarkChanProdCons0           273.00       254.00   -6.96%
BenchmarkChanProdCons0-2         599.00       400.00  -33.22%
BenchmarkChanProdCons0-4        1168.00       659.00  -43.58%
BenchmarkChanProdCons0-8        2831.00      1057.00  -62.66%
BenchmarkChanProdCons0-16       4197.00      1037.00  -75.29%
BenchmarkChanProdCons10          150.00       140.00   -6.67%
BenchmarkChanProdCons10-2        607.00       268.00  -55.85%
BenchmarkChanProdCons10-4       1137.00       404.00  -64.47%
BenchmarkChanProdCons10-8       2115.00       828.00  -60.85%
BenchmarkChanProdCons10-16      4283.00       855.00  -80.04%
BenchmarkChanProdCons100         117.00       110.00   -5.98%
BenchmarkChanProdCons100-2       558.00       218.00  -60.93%
BenchmarkChanProdCons100-4       722.00       287.00  -60.25%
BenchmarkChanProdCons100-8      1840.00       431.00  -76.58%
BenchmarkChanProdCons100-16     3394.00       448.00  -86.80%
BenchmarkChanProdConsWork0      2014.00      1996.00   -0.89%
BenchmarkChanProdConsWork0-2    1207.00      1127.00   -6.63%
BenchmarkChanProdConsWork0-4    1913.00       611.00  -68.06%
BenchmarkChanProdConsWork0-8    3016.00       949.00  -68.53%
BenchmarkChanProdConsWork0-16   4320.00      1154.00  -73.29%
BenchmarkChanProdConsWork10     1906.00      1897.00   -0.47%
BenchmarkChanProdConsWork10-2   1123.00      1033.00   -8.01%
BenchmarkChanProdConsWork10-4   1076.00       571.00  -46.93%
BenchmarkChanProdConsWork10-8   2748.00      1096.00  -60.12%
BenchmarkChanProdConsWork10-16  4600.00      1105.00  -75.98%
BenchmarkChanProdConsWork100    1884.00      1852.00   -1.70%
BenchmarkChanProdConsWork100-2  1235.00      1146.00   -7.21%
BenchmarkChanProdConsWork100-4  1217.00       619.00  -49.14%
BenchmarkChanProdConsWork100-8  1534.00       509.00  -66.82%
BenchmarkChanProdConsWork100-16 4126.00       918.00  -77.75%
BenchmarkSyscall                  34.40        33.30   -3.20%
BenchmarkSyscall-2               160.00       121.00  -24.38%
BenchmarkSyscall-4               131.00       136.00   +3.82%
BenchmarkSyscall-8               139.00       131.00   -5.76%
BenchmarkSyscall-16              161.00       168.00   +4.35%
BenchmarkSyscallWork             950.00       950.00   +0.00%
BenchmarkSyscallWork-2           481.00       480.00   -0.21%
BenchmarkSyscallWork-4           268.00       270.00   +0.75%
BenchmarkSyscallWork-8           156.00       169.00   +8.33%
BenchmarkSyscallWork-16          188.00       184.00   -2.13%
BenchmarkSemaSyntNonblock         36.40        35.60   -2.20%
BenchmarkSemaSyntNonblock-2       81.40        45.10  -44.59%
BenchmarkSemaSyntNonblock-4      126.00       108.00  -14.29%
BenchmarkSemaSyntNonblock-8      112.00       112.00   +0.00%
BenchmarkSemaSyntNonblock-16     110.00       112.00   +1.82%
BenchmarkSemaSyntBlock            35.30        35.30   +0.00%
BenchmarkSemaSyntBlock-2         118.00       124.00   +5.08%
BenchmarkSemaSyntBlock-4         105.00       108.00   +2.86%
BenchmarkSemaSyntBlock-8         101.00       111.00   +9.90%
BenchmarkSemaSyntBlock-16        112.00       118.00   +5.36%
BenchmarkSemaWorkNonblock        810.00       811.00   +0.12%
BenchmarkSemaWorkNonblock-2      476.00       414.00  -13.03%
BenchmarkSemaWorkNonblock-4      238.00       228.00   -4.20%
BenchmarkSemaWorkNonblock-8      140.00       126.00  -10.00%
BenchmarkSemaWorkNonblock-16     117.00       116.00   -0.85%
BenchmarkSemaWorkBlock           810.00       811.00   +0.12%
BenchmarkSemaWorkBlock-2         454.00       466.00   +2.64%
BenchmarkSemaWorkBlock-4         243.00       241.00   -0.82%
BenchmarkSemaWorkBlock-8         145.00       137.00   -5.52%
BenchmarkSemaWorkBlock-16        132.00       123.00   -6.82%
BenchmarkContendedSemaphore      123.00       102.00  -17.07%
BenchmarkContendedSemaphore-2     34.80        34.90   +0.29%
BenchmarkContendedSemaphore-4     34.70        34.80   +0.29%
BenchmarkContendedSemaphore-8     34.70        34.70   +0.00%
BenchmarkContendedSemaphore-16    34.80        34.70   -0.29%
BenchmarkMutex                    26.80        26.00   -2.99%
BenchmarkMutex-2                 108.00        45.20  -58.15%
BenchmarkMutex-4                 103.00       127.00  +23.30%
BenchmarkMutex-8                 109.00       147.00  +34.86%
BenchmarkMutex-16                102.00       152.00  +49.02%
BenchmarkMutexSlack               27.00        26.90   -0.37%
BenchmarkMutexSlack-2            149.00       165.00  +10.74%
BenchmarkMutexSlack-4            121.00       209.00  +72.73%
BenchmarkMutexSlack-8            101.00       158.00  +56.44%
BenchmarkMutexSlack-16            97.00       129.00  +32.99%
BenchmarkMutexWork               792.00       794.00   +0.25%
BenchmarkMutexWork-2             407.00       409.00   +0.49%
BenchmarkMutexWork-4             220.00       209.00   -5.00%
BenchmarkMutexWork-8             267.00       160.00  -40.07%
BenchmarkMutexWork-16            315.00       300.00   -4.76%
BenchmarkMutexWorkSlack          792.00       793.00   +0.13%
BenchmarkMutexWorkSlack-2        406.00       404.00   -0.49%
BenchmarkMutexWorkSlack-4        225.00       212.00   -5.78%
BenchmarkMutexWorkSlack-8        268.00       136.00  -49.25%
BenchmarkMutexWorkSlack-16       300.00       300.00   +0.00%
BenchmarkRWMutexWrite100          27.10        27.00   -0.37%
BenchmarkRWMutexWrite100-2        33.10        40.80  +23.26%
BenchmarkRWMutexWrite100-4       113.00        88.10  -22.04%
BenchmarkRWMutexWrite100-8       119.00        95.30  -19.92%
BenchmarkRWMutexWrite100-16      148.00       109.00  -26.35%
BenchmarkRWMutexWrite10           29.60        29.40   -0.68%
BenchmarkRWMutexWrite10-2        111.00        61.40  -44.68%
BenchmarkRWMutexWrite10-4        270.00       208.00  -22.96%
BenchmarkRWMutexWrite10-8        204.00       185.00   -9.31%
BenchmarkRWMutexWrite10-16       261.00       190.00  -27.20%
BenchmarkRWMutexWorkWrite100    1040.00      1036.00   -0.38%
BenchmarkRWMutexWorkWrite100-2   593.00       580.00   -2.19%
BenchmarkRWMutexWorkWrite100-4   470.00       365.00  -22.34%
BenchmarkRWMutexWorkWrite100-8   468.00       289.00  -38.25%
BenchmarkRWMutexWorkWrite100-16  604.00       374.00  -38.08%
BenchmarkRWMutexWorkWrite10      951.00       951.00   +0.00%
BenchmarkRWMutexWorkWrite10-2   1001.00       928.00   -7.29%
BenchmarkRWMutexWorkWrite10-4   1555.00      1006.00  -35.31%
BenchmarkRWMutexWorkWrite10-8   2085.00      1171.00  -43.84%
BenchmarkRWMutexWorkWrite10-16  2082.00      1614.00  -22.48%

R=rsc, iant, msolo, fw, iant
CC=golang-dev
https://golang.org/cl/4711045