From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga17.intel.com (mga17.intel.com [192.55.52.151]) by dpdk.org (Postfix) with ESMTP id 98C561C699 for ; Fri, 13 Apr 2018 18:10:19 +0200 (CEST) X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 13 Apr 2018 09:10:18 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.48,446,1517904000"; d="scan'208";a="33464816" Received: from aburakov-mobl.ger.corp.intel.com (HELO [10.237.220.128]) ([10.237.220.128]) by orsmga008.jf.intel.com with ESMTP; 13 Apr 2018 09:10:17 -0700 To: Adrien Mazarguil Cc: dev@dpdk.org References: <20180413153749.28208-1-adrien.mazarguil@6wind.com> <20180413155417.29643-1-adrien.mazarguil@6wind.com> From: "Burakov, Anatoly" Message-ID: <0d2c152d-2b62-3772-8a40-05d73bb2ef74@intel.com> Date: Fri, 13 Apr 2018 17:10:17 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180413155417.29643-1-adrien.mazarguil@6wind.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [dpdk-dev] [PATCH v2 1/2] eal: fix undefined behavior in fbarray X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 13 Apr 2018 16:10:20 -0000 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 > > 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 > > Signed-off-by: Adrien Mazarguil > --- Acked-by: Anatoly Burakov -- Thanks, Anatoly