FFTW planner is not thread safe.

[KristofferC]

Moved from JuliaLang/julia#17972

---

Having fft/bfft/ifft/etc. calls in a threaded region exposes the fact that the planning portion of the fft call is not thread-safe in fftw. Below is a potential patch to dft.jl if you just want to lock the planner.

global planner_mutex = nothing

function __init__()
    global planner_mutex = Base.Threads.Mutex()
end

# implementations only need to provide plan_X(x, region)
# for X in (:fft, :bfft, ...):
for f in (:fft, :bfft, :ifft, :fft!, :bfft!, :ifft!, :rfft)
    pf = Symbol("plan_", f)
    @eval begin
        function $f(x::AbstractArray)
            Base.Threads.lock!(planner_mutex)
            the_plan = $pf(x)
            Base.Threads.unlock!(planner_mutex)
            the_plan * x
        end
        function $f(x::AbstractArray, region)
            Base.Threads.lock!(planner_mutex)
            the_plan = $pf(x, region)
            Base.Threads.unlock!(planner_mutex)
            the_plan * x
        end
        function $pf(x::AbstractArray; kws...)
            Base.Threads.lock!(planner_mutex)
            the_plan = $pf(x, 1:ndims(x); kws...)
            Base.Threads.unlock!(planner_mutex)
            the_plan
        end
    end
end

[stevengj]

You could call fftw_make_planner_thread_safe instead, although it basically does the same thing.

If you are implementing your own locks, note that you also need to put a lock around the fftw_destroy_plan call.

[dlfivefifty]

This would be nice to have for ApproxFun, as I think it is the one thing used that is not thread-safe.