df206c6e77
If getSiginfo does not know how to determine the PC, it will call
runtime_callers. That can happen in a thread that was started by
non-Go code, in which case the TLS variable g will not be set, in
which case runtime_lock will crash.
Avoid the problem by using atomic operations for the lock. This is OK
since creating a backtrace state is fast and never blocks.
The test case is TestCgoExternalThreadSIGPROF in the runtime package
on a system that getSiginfo doesn't handle specially.
Updates golang/go#20931
Reviewed-on: https://go-review.googlesource.com/50650
From-SVN: r250439
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