What steps will reproduce the problem?
src/pkg/bytes$ hg summary
11226:70e914beb409 tip
src/pkg/bytes$ GOMAXPROCS=17 go test

unexpected fault address 0x4002180001 
throw: fault 
[signal 0xb code=0x1 addr=0x4002180001 pc=0x41fe0b] 

When run under ThreadSanitizer
(http://code.google.com/p/data-race-test/wiki/ThreadSanitizerGo) the following report is
produced:

INFO: T2 has been created by T0 at this point: {{{
    #0  main..do src/cmd/go/build.go:452
    #1  main.runTest src/cmd/go/test.go:297
    #2  main.main src/cmd/go/main.go:111
    #3  runtime.main src/pkg/runtime/proc.c:238
}}}
INFO: T1 has been created by T0 at this point: {{{
    #0  main..do src/cmd/go/build.go:452
    #1  main.runTest src/cmd/go/test.go:297
    #2  main.main src/cmd/go/main.go:111
    #3  runtime.main src/pkg/runtime/proc.c:238
}}}
WARNING: Possible data race during write of size 1 at 0xf84009f9a0: {{{
   T2 (L{}):
    #1  main..build src/cmd/go/build.go:881
    #1  main..build src/cmd/go/build.go:488
    #2  main._func_004 src/cmd/go/build.go:414
    #3  main._func_005 src/cmd/go/build.go:463
    #4  runtime.goexit src/pkg/runtime/proc.c:266
  Concurrent write(s) happened at (OR AFTER) these points:
   T1 (L{}):
    #0  main..mkdir src/cmd/go/build.go:881
    #1  main..build src/cmd/go/build.go:488
    #2  main._func_004 src/cmd/go/build.go:414
    #3  main._func_005 src/cmd/go/build.go:463
    #4  runtime.goexit src/pkg/runtime/proc.c:266
}}}