Concurrency bugs in the latest release

[Octogonapus]

With the release of v1.2.3, I am now seeing a lot of these errors:

error in running finalizer: ErrorException("task switch not allowed from inside gc finalizer")
error in running finalizer: ErrorException("val already in a list")

We use FFTW on multiple threads. Is this a supported use case?

I would have liked to attach a small example to reproduce the problem, but I can't figure one out. Maybe it would help if others commented with ideas on how these errors could happen.

[stevengj]

No, this was not intentional. We were trying to fix a deadlock in the finalizers in #160, and apparently I introduced another problem. 😢

@vtjnash, if we can't access shared resources in a finalizer that are protected by a lock, that will be frustrating to work around. Any ideas?

[stevengj]

(Separate from this bug, I wouldn't recommend creating and destroying lots of plans from different threads for performance reasons. Create your plans once with plan_fft and then re-use them in parallel as much as you want.)

[stevengj]

I can reproduce the problem: run the following with JULIA_NUM_THREADS=8:

using FFTW
FFTW.set_num_threads(1)
Threads.@threads for i = 1:10000
    a = rand(100)
    fft(a)
    GC.gc()
end

[stevengj]

@Octogonapus, can you try out #162 and see if it fixes your problem? (My testcase above passes with that patch.)

[stevengj]

FFTW 1.2.4 should be merged soon and includes the fix (JuliaRegistries/General#19855)