[dpdk-users] Broken hash table functions with CFLAGS -O3

Pablo de Lara pablo.de.lara.guarch at intel.com
Mon Jan 18 15:25:20 CET 2016

> -----Original Message-----
> From: users [mailto:users-bounces at dpdk.org] On Behalf Of Dmitriy Yakovlev
> Sent: Friday, January 15, 2016 1:20 AM
> To: users at dpdk.org
> Subject: [dpdk-users] Broken hash table functions with CFLAGS -O3
> Hello.
> I trying to use hash tables for adding struct ip:port as key.
> When trying to add duplicated key it's successfuly added second time, not
> always, but sometimes.
> Please see this youtube video, where I'm demonstrating this bug:
> https://www.youtube.com/watch?v=vsXVxISpmnU
> Code on github: https://github.com/BombermaG/bug-app
> When using CFLAGS -O2 this problem cannot be repeated.
> It's problem with -O3 flag and I must use O2 or how can I solve it?
> Thank you for helping.
> (Sorry if this message double posting).
> Spec info. Tell me, if you need more info.
> dpdk 2.2.0

Hello Dmitriy,

Thanks for the detailed report. I am able to reproduce the issue with
gcc 4.7 and 4.8. The problem is that the signature sometimes changes,
due to a different key and therefore the bucket where it should be stored
is the wrong one.

Actually, the hash table is fine. The issue is that your key is a structure
of a 4-byte and 2-byte integers, so your key size should be 6, but instead,
it is 8, because sizeof(struct...) includes padding to have it multiple of 4.
Most times, that padding is 0, so it is fine, but when that padding is not 0,
the key is different and then it looks like you are adding a duplicate
(but actually you are adding a different key!).
So, either add padding manually in the key or manually set the key size to 6.

> Linux 3.10.0-229.el7.x86_64 #1 SMP Fri Mar 6 11:36:42 UTC 2015 x86_64
> x86_64 x86_64 GNU/Linux
> gcc -v
> Using built-in specs.
> COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
> Target: x86_64-redhat-linux
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --
> infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
> --enable-bootstrap --enable-shared --enable-threads=posix --enable-
> checking=release --with-system-zlib --enable-__cxa_atexit --disable-
> libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id -
> -with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-
> c++,java,fortran,ada,go,lto
> --enable-plugin --enable-initfini-array --disable-libgcj --with-
> isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-
> install
> --with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-
> linux/cloog-install
> --enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 -
> -build=x86_64-redhat-linux Thread model: posix gcc version 4.8.5 20150623
> (Red Hat 4.8.5-4) (GCC)
> Intel(R) Xeon(R) CPU E5-2643 v2 @ 3.50GHz

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