I'm not sure whether this is due to a change in the builder or a change in the runtime or standard library, but lockedfile.TestSpuriousEDEADLK has been failing sporadically on the plan9-arm builder, apparently due to the failure of a *lockedfile.File to provide the expected mutual-exclusion when open for writing.

The first such failure occurred at CL 260137, but it's not obvious to me how that could be related. (The plan9-386-0intro builder started working again around then — do the -386-0intro and -arm builders share an underlying filesystem?)

CC @millerresearch @fhs @0intro

--- FAIL: TestSpuriousEDEADLK (0.04s)
    lockedfile_test.go:255: Edit B unexpectedly did not block

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