I don't have a specific failure due to this, but was looking at the code for allocation of the P, M, and G structs within proc.c, and it looks to me like the size being used to allocate a P struct for the runtime_allp array is incorrect. The call to runtime_mallocgc for a new P struct is using the size of a pointer instead of the size of the struct. The line of code in procresize in libgo/runtime/proc.c looks like this:

p = (P*)runtime_mallocgc(sizeof(*p), 0, FlagNoInvokeGC);

Since it should be allocating a P struct I think it should be sizeof(P) instead.