Parallel.for going to sleep - f#

Question

I am running some code written in F# with mono and I have seen that after some time running (weeks), some process are in sleep mode. Using strace as strace -p <pid> I get
Process 38756 attached - interrupt to quit
futex(0x272ee9c, FUTEX_WAIT_PRIVATE, 1, NULL
Which as far as I understand it means that some part is awaiting for something (other threads).
So I continued searching the problem with gdb attach <pid> in which I can see that there are 4 threads as:
(gdb) info threads
4 Thread 0x7ff06a3ff700 (LWP 38760) 0x0000003b3c80b5bc in pthread_cond_wait##GLIBC_2.3.2 () from /lib64/libpthread.so.0
3 Thread 0x7ff06aa17700 (LWP 38769) 0x0000003b3c80d930 in sem_wait () from /lib64/libpthread.so.0
2 Thread 0x7ff0677ff700 (LWP 38789) 0x0000003b3c80b5bc in pthread_cond_wait##GLIBC_2.3.2 () from /lib64/libpthread.so.0
* 1 Thread 0x7ff070a07760 (LWP 38756) 0x0000003b3c80b5bc in pthread_cond_wait##GLIBC_2.3.2 () from /lib64/libpthread.so.0
On thread 1 using where I can see that the place where it is not working is a Parallel.For().
From my part, the code where the process goes to sleep is:
let sinMod t (difRay:Vector) (fA:(float*Vector*float)[]) =
let duepi = 2.*PI
Array.fold(fun acc x -> let freq, amplitude, phase = fA.[x]
acc+sin(duepi*freq*t+phase)*(amplitude*difRay)) 0. [|0..fA.Length-1|]
let NextFunc (var1:type1,var2:type2,var3:type3) =
// do some stuff
...
// some context
let time = snd var3
let inputArray = var.SomeArray // float[]
// the function that doesn't works
Parallel.For(0, (inputArray .Length), fun i -> (inputArray .[i] <- inputArray .[i]+(ww*SINMOD time.[i])) ) |> ignore
inputArray // return from NextFunc
I really don't have any idea where is the problem. Sometimes the process goes to sleep and others this is not happening. I don't know if this can have any relation, but the length of the parallel.for is 40960
Why is the program going to sleep?
Thank you
EDIT:
I haven't specified why I believe the problem is happening in a function like I said. Using gdb I obtain:
(gdb) where
#0 0x0000003b3c80b5bc in pthread_cond_wait##GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1 0x000000000060cd0d in mono_os_cond_wait (cond=0x272ee98, mutex=0x272ee70, timeout_ms=4294967295) at ../../mono/utils/mono-os-mutex.h:105
#2 mono_os_cond_timedwait (cond=0x272ee98, mutex=0x272ee70, timeout_ms=4294967295) at ../../mono/utils/mono-os-mutex.h:120
#3 0x000000000060f0c8 in _wapi_handle_timedwait_signal_handle (handle=0x410, timeout=4294967295, alertable=1, poll=0, alerted=0x7fff27d2a6cc)
at handles.c:1554
#4 0x00000000006245ea in wapi_WaitForSingleObjectEx (handle=0x410, timeout=4294967295, alertable=1) at wait.c:173
#5 0x00000000005b01f2 in ves_icall_System_Threading_Monitor_Monitor_wait (obj=0x7fed09df4cc0, ms=4294967295) at monitor.c:1294
#6 0x00007ff06806385e in System.Threading.Monitor:Wait (obj=<type 'exceptions.ValueError'>
zero length field name in format
140656049605824, millisecondsTimeout=-1) at /root/mono-4.3.2/mcs/class/corlib/System.Threading/Monitor.cs:148
#7 0x00007ff067e69dc2 in System.Threading.ManualResetEventSlim:Wait (this=..., millisecondsTimeout=-1, cancellationToken=0)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/ManualResetEventSlim.cs:669
#8 0x00007ff067e8544e in System.Threading.Tasks.Task:SpinThenBlockingWait (this=..., millisecondsTimeout=-1, cancellationToken=0)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:3326
#9 0x00007ff067e8255d in System.Threading.Tasks.Task:InternalRunSynchronously (this=..., scheduler=..., waitForCompletion=true)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:1231
#10 0x00007ff067e823c8 in System.Threading.Tasks.Task:RunSynchronously (this=..., scheduler=...)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:1169
#11 0x00007ff067e77c03 in System.Threading.Tasks.Parallel:ForWorker<TLocal_REF> (fromInclusive=0, toExclusive=40960, parallelOptions=..., body=...,
bodyWithState=..., bodyWithLocal=..., localInit=..., localFinally=...)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/Tasks/Parallel.cs:1273
#12 0x00007ff067e76b8a in System.Threading.Tasks.Parallel:For (fromInclusive=0, toExclusive=40960, body=...)
at /root/mono-4.3.2/external/referencesource/mscorlib/system/threading/Tasks/Parallel.cs:441
#13 0x00000000402c11ee in ?? ()
#14 0x00007fff27d2aec0 in ?? ()
#15 0x00007ff0167eab00 in ?? ()
#16 0x0000000000000016 in ?? ()
#17 0x00007fef394db070 in ?? ()
#18 0x0000000000000000 in ?? ()
(gdb) p mono_pmip(0x00000000402c11ee)
$1 =
0x29ae0f0 " ShadingNoise:PhaseModulation (Types.types/Ray,Types.types/Intersection,System.Tuple`2<System.Tuple`3<double, Types.Algebra/Vector, double>[], double[]>) + 0x27e (0x402c0f70 0x402c1238) [0x273b080 - m"...
(gdb)
Where to specify, ShadingNoise:PhaseModulation is the function that I have defined as NextFunc in a simplified way. just in case, the original function it's here