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To: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Cc: dev@dpdk.org
References: <20180413153749.28208-1-adrien.mazarguil@6wind.com>
<20180413155417.29643-1-adrien.mazarguil@6wind.com>
From: "Burakov, Anatoly" <anatoly.burakov@intel.com>
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Date: Fri, 13 Apr 2018 17:10:17 +0100
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Subject: Re: [dpdk-dev] [PATCH v2 1/2] eal: fix undefined behavior in fbarray
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On 13-Apr-18 4:56 PM, Adrien Mazarguil wrote:
> According to GCC documentation [1], the __builtin_clz() family of functions
> yield undefined behavior when fed a zero value. There is one instance in
> the fbarray code where this can occur.
>
> Clang (at least version 3.8.0-2ubuntu4) seems much more sensitive to this
> than GCC and yields random results when compiling optimized code, as shown
> below:
>
> #include <stdio.h>
>
> int main(void)
> {
> volatile unsigned long long moo;
> int x;
>
> moo = 0;
> x = __builtin_clzll(moo);
> printf("%d\n", x);
> return 0;
> }
>
> $ gcc -O3 -o test test.c && ./test
> 63
> $ clang -O3 -o test test.c && ./test
> 1742715559
> $ clang -O0 -o test test.c && ./test
> 63
>
> Even 63 can be considered an unexpected result given the number of leading
> zeroes should be the full width of the underlying type, i.e. 64.
>
> In practice it causes find_next_n() to sometimes return negative values
> interpreted as errors by caller functions, which prevents DPDK applications
> from starting due to inability to find free memory segments:
>
> # testpmd [...]
> EAL: Detected 32 lcore(s)
> EAL: Detected 2 NUMA nodes
> EAL: No free hugepages reported in hugepages-1048576kB
> EAL: Multi-process socket /var/run/.rte_unix
> EAL: eal_memalloc_alloc_seg_bulk(): couldn't find suitable memseg_list
> EAL: FATAL: Cannot init memory
>
> EAL: Cannot init memory
>
> PANIC in main():
> Cannot init EAL
> 4: [./build/app/testpmd(_start+0x29) [0x462289]]
> 3: [/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)
> [0x7f19d54fc830]]
> 2: [./build/app/testpmd(main+0x8a3) [0x466193]]
> 1: [./build/app/testpmd(__rte_panic+0xd6) [0x4efaa6]]
> Aborted
>
> This problem appears with commit 66cc45e293ed ("mem: replace memseg with
> memseg lists") however the root cause is introduced by a prior patch.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>
> Fixes: c44d09811b40 ("eal: add shared indexed file-backed array")
> Cc: Anatoly Burakov <anatoly.burakov@intel.com>
>
> Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> ---
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
--
Thanks,
Anatoly