livelock causes librte_distributor unit test to hang

https://bugs.dpdk.org/show_bug.cgi?id=316

            Bug ID: 316
           Summary: livelock causes librte_distributor unit test to hang
           Product: DPDK
           Version: 19.08
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: other
          Assignee: [email protected]
          Reporter: [email protected]

Target Milestone: ---

Issue first encountered in Travis CI.

The meson unit test distributor_autotest randomly times out. Normally this test
finishes in less than 1/2 a second, so running to 10 seconds and timing out is
a big jump in run time.

I was able to reproduce by running:

while sudo sh -c "echo 'distributor_autotest' | ./build/app/test/dpdk-test"; do :; done

It runs a couple of times fine showing output and showing progress, but then at some point after a couple of seconds it just stops - no longer getting any output. It just sits there with no further output. I let it sit there for a whole minute and nothing happens. So I attach gdb to try to figure out what is happening. One thread seems to be stuck on a while loop, see lib/librte_distributor/rte_distributor.c:310.

I looked at the assembly code (layout asm, ni) and I saw these four lines below (which correspond to the while loop) being executed repeatedly and indefinitely. It looks like this thread is waiting for the variable bufptr64[0] to change state.

0xa064d0 <release+32>   pause
0xa064d2 <release+34>   mov    0x3840(%rdx),%rax
0xa064d9 <release+41>   test   $0x1,%al
0xa064db <release+43>   je     0xa064d0 <release+32>

While the first thread is waiting on bufptr64[0] to change state, there is another thread that is also stuck on another while loop on lib/librte_distributor/rte_distributor.c:53. It seems that this thread is stuck waiting for retptr64 to change state. Corresponding assembly being executed indefinitely:

0xa06de0 <rte_distributor_request_pkt_v1705+592> mov    0x38c0(%r8),%rax
0xa06de7 <rte_distributor_request_pkt_v1705+599> test   $0x1,%al
0xa06de9 <rte_distributor_request_pkt_v1705+601> je     0xa06bbd

<rte_distributor_request_pkt_v1705+45>
0xa06def <rte_distributor_request_pkt_v1705+607> nop
0xa06df0 <rte_distributor_request_pkt_v1705+608> pause 0xa06df2 <rte_distributor_request_pkt_v1705+610> rdtsc

0xa06df4 <rte_distributor_request_pkt_v1705+612> mov    %rdx,%r10
0xa06df7 <rte_distributor_request_pkt_v1705+615> shl    $0x20,%r10
0xa06dfb <rte_distributor_request_pkt_v1705+619> mov    %eax,%eax
0xa06dfd <rte_distributor_request_pkt_v1705+621> or     %r10,%rax
0xa06e00 <rte_distributor_request_pkt_v1705+624> lea    0x64(%rax),%r10
0xa06e04 <rte_distributor_request_pkt_v1705+628> jmp    0xa06e12

<rte_distributor_request_pkt_v1705+642>
0xa06e06 <rte_distributor_request_pkt_v1705+630> nopw %cs:0x0(%rax,%rax,1)
0xa06e10 <rte_distributor_request_pkt_v1705+640> pause 0xa06e12 <rte_distributor_request_pkt_v1705+642> rdtsc

0xa06e14 <rte_distributor_request_pkt_v1705+644> shl    $0x20,%rdx
0xa06e18 <rte_distributor_request_pkt_v1705+648> mov    %eax,%eax
0xa06e1a <rte_distributor_request_pkt_v1705+650> or     %rdx,%rax
0xa06e1d <rte_distributor_request_pkt_v1705+653> cmp    %rax,%r10
0xa06e20 <rte_distributor_request_pkt_v1705+656> ja     0xa06e10

<rte_distributor_request_pkt_v1705+640>
0xa06e22 <rte_distributor_request_pkt_v1705+658> jmp 0xa06de0
<rte_distributor_request_pkt_v1705+592>

My guess is that these threads are interdependent, so one thread is waiting for the other thread to change the state of the control variable. I can't say for sure if this is what is happening or why the these variables don't change state, so I would like ask someone who is more familiar with this particular code to take a look

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