f2e5e85824
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")
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
DPDK is a set of libraries and drivers for fast packet processing. It supports many processor architectures and both FreeBSD and Linux. The DPDK uses the Open Source BSD-3-Clause license for the core libraries and drivers. The kernel components are GPL-2.0 licensed. Please check the doc directory for release notes, API documentation, and sample application information. For questions and usage discussions, subscribe to: users@dpdk.org Report bugs and issues to the development mailing list: dev@dpdk.org