In my code I have loop in which I construct and over determined linear system and try to solve it:

    #pragma omp parallel for
    for (int i = 0; i &lt; n[0]+1; i++) {
        for (int j = 0; j &lt; n[1]+1; j++) {
            for (int k = 0; k &lt; n[2]+1; k++) {
                arma::mat A(max_points, 2);
                arma::mat y(max_points, 1);
                // initialize A and y
    
                arma::vec solution = solve(A,y);
            }
        }
    }

Sometimes, quite randomly the program hangs or the results in the solution vector are NaN. And if I put do this:

    arma::vec solution;
    #pragma omp critical 
    {
        solution = solve(weights*A,weights*y);
    }

then these problem don&#39;t seem to happen anymore.

When it hangs, it does so because some threads are waiting at the OpenMP barrier:

    Thread 2 (Thread 0x7fe4325a5700 (LWP 39839)):
    #0  0x00007fe44d3c2084 in gomp_team_barrier_wait_end () from /usr/lib64/gcc-4.9.2/lib64/gcc/x86_64-redhat-linux-gnu/4.9.2/libgomp.so.1
    #1  0x00007fe44d3bf8c2 in gomp_thread_start () at ../.././libgomp/team.c:118
    #2  0x0000003f64607851 in start_thread () from /lib64/libpthread.so.0
    #3  0x0000003f642e890d in clone () from /lib64/libc.so.6

And the other threads are stuck inside Armadillo: 

    Thread 1 (Thread 0x7fe44afe2e60 (LWP 39800)):
    #0  0x0000003ee541f748 in dscal_ () from /usr/lib64/libblas.so.3
    #1  0x00007fe44c0d3666 in dlarfp_ () from /usr/lib64/atlas/liblapack.so.3
    #2  0x00007fe44c058736 in dgelq2_ () from /usr/lib64/atlas/liblapack.so.3
    #3  0x00007fe44c058ad9 in dgelqf_ () from /usr/lib64/atlas/liblapack.so.3
    #4  0x00007fe44c059a32 in dgels_ () from /usr/lib64/atlas/liblapack.so.3
    #5  0x00007fe44f09fb3d in bool arma::auxlib::solve_ud&lt;double, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt; &gt;(arma::Mat&lt;double&gt;&amp;, arma::Mat&lt;double&gt;&amp;, arma::Base&lt;double, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt; &gt; const&amp;) () at /usr/include/armadillo_bits/lapack_wrapper.hpp:677
    #6  0x00007fe44f0a0f87 in arma::Col&lt;double&gt;::Col&lt;arma::Glue&lt;arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt;, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt;, arma::glue_solve&gt; &gt;(arma::Base&lt;double, arma::Glue&lt;arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt;, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt;, arma::glue_solve&gt; &gt; const&amp;) ()
    at /usr/include/armadillo_bits/glue_solve_meat.hpp:39

As you can see from the stacktrace my version of Armadillo uses atlas. And according to this documentation atlas seems to be thread safe: ftp://lsec.cc.ac.cn/netlib/atlas/faq.html#tsafe

**Update 9/11/2015**

I finally got some time to run more tests, based on the suggestions of Vladimir F.

When I compile armadillo with ATLAS&#39;s BLAS, I&#39;m still able to reproduce then hangs and the NaNs. When it hangs, the only thing that changes in the stacktrace is the call to BLAS:

    #0  0x0000003fa8054718 in ATL_dscal_xp1yp0aXbX@plt () from /usr/lib64/atlas/libatlas.so.3
    #1  0x0000003fb05e7666 in dlarfp_ () from /usr/lib64/atlas/liblapack.so.3
    #2  0x0000003fb0576a61 in dgeqr2_ () from /usr/lib64/atlas/liblapack.so.3
    #3  0x0000003fb0576e06 in dgeqrf_ () from /usr/lib64/atlas/liblapack.so.3
    #4  0x0000003fb056d7d1 in dgels_ () from /usr/lib64/atlas/liblapack.so.3
    #5  0x00007ff8f3de4c34 in void arma::lapack::gels&lt;double&gt;(char*, int*, int*, int*, double*, int*, double*, int*, double*, int*, int*) () at /usr/include/armadillo_bits/lapack_wrapper.hpp:677
    #6  0x00007ff8f3de1787 in bool arma::auxlib::solve_od&lt;double, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt; &gt;(arma::Mat&lt;double&gt;&amp;, arma::Mat&lt;double&gt;&amp;, arma::Base&lt;double, arma::Glue&lt;arma::Mat&lt;double&gt;, arma::Mat&lt;double&gt;, arma::glue_times&gt; &gt; const&amp;) () at /usr/include/armadillo_bits/auxlib_meat.hpp:3434


Compiling without ATLAS, only with netlib BLAS and LAPACK, I was able to reproduce the NaNs but not the hangs.

In both cases, surrounding `solve()` with `#pragma` omp critical I have no problems at all

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Is armadillo solve() thread safe?