From 0c04218728893cda960b3ccd885bab9d1c01a033 Mon Sep 17 00:00:00 2001 From: Michael Jeanson Date: Fri, 29 Jul 2022 10:43:15 -0400 Subject: [PATCH] fix: Unify possible CPU number fallback The MUSL specific fallback to get the number of possible CPUs in the system has the same issue with hot-unplugged CPUs as the Glibc implementation we worked around by using the possible CPU mask from sysfs. To address this, unify our fallback code across all C libraries to get the maximum CPU id from the directories in "/sys/devices/system/cpu". Change-Id: I5541742dc1de8e011a942880825fa88c656f0905 Signed-off-by: Michael Jeanson Signed-off-by: Mathieu Desnoyers --- libringbuffer/smp.c | 83 +++++++++++++++++++++++---------------------- 1 file changed, 42 insertions(+), 41 deletions(-) diff --git a/libringbuffer/smp.c b/libringbuffer/smp.c index b6c2962b..a507df32 100644 --- a/libringbuffer/smp.c +++ b/libringbuffer/smp.c @@ -23,55 +23,47 @@ #define _LGPL_SOURCE #include #include +#include #include #include #include #include #include #include +#include +#include #include "smp.h" #include "usterr-signal-safe.h" -int __num_possible_cpus; - -#if (defined(__GLIBC__) || defined( __UCLIBC__)) -int get_num_possible_cpus_fallback(void) -{ - /* On Linux, when some processors are offline - * _SC_NPROCESSORS_CONF counts the offline - * processors, whereas _SC_NPROCESSORS_ONLN - * does not. If we used _SC_NPROCESSORS_ONLN, - * getcpu() could return a value greater than - * this sysconf, in which case the arrays - * indexed by processor would overflow. - */ - return sysconf(_SC_NPROCESSORS_CONF); -} +#define __max(a,b) ((a)>(b)?(a):(b)) -#else +int __num_possible_cpus; /* - * The MUSL libc implementation of the _SC_NPROCESSORS_CONF sysconf does not - * return the number of configured CPUs in the system but relies on the cpu - * affinity mask of the current task. + * As a fallback to parsing the CPU mask in "/sys/devices/system/cpu/possible", + * iterate on all the folders in "/sys/devices/system/cpu" that start with + * "cpu" followed by an integer, keep the highest CPU id encountered during + * this iteration and add 1 to get a number of CPUs. + * + * Then get the value from sysconf(_SC_NPROCESSORS_CONF) as a fallback and + * return the highest one. + * + * On Linux, using the value from sysconf can be unreliable since the way it + * counts CPUs varies between C libraries and even between versions of the same + * library. If we used it directly, getcpu() could return a value greater than + * this sysconf, in which case the arrays indexed by processor would overflow. + * + * As another example, the MUSL libc implementation of the _SC_NPROCESSORS_CONF + * sysconf does not return the number of configured CPUs in the system but + * relies on the cpu affinity mask of the current task. * - * So instead we use a strategy similar to GLIBC's, counting the cpu - * directories in "/sys/devices/system/cpu" and fallback on the value from - * sysconf if it fails. + * Returns 0 or less on error. */ - -#include -#include -#include -#include -#include - -#define __max(a,b) ((a)>(b)?(a):(b)) - int get_num_possible_cpus_fallback(void) { - int count = 0; + long max_cpuid = -1; + DIR *cpudir; struct dirent *entry; @@ -80,31 +72,40 @@ int get_num_possible_cpus_fallback(void) goto end; /* - * Count the number of directories named "cpu" followed by and - * integer. This is the same strategy as glibc uses. + * Iterate on all directories named "cpu" followed by an integer. */ while ((entry = readdir(cpudir))) { if (entry->d_type == DT_DIR && strncmp(entry->d_name, "cpu", 3) == 0) { char *endptr; - unsigned long cpu_num; + long cpu_id; - cpu_num = strtoul(entry->d_name + 3, &endptr, 10); - if ((cpu_num < ULONG_MAX) && (endptr != entry->d_name + 3) + cpu_id = strtol(entry->d_name + 3, &endptr, 10); + if ((cpu_id < LONG_MAX) && (endptr != entry->d_name + 3) && (*endptr == '\0')) { - count++; + if (cpu_id > max_cpuid) + max_cpuid = cpu_id; } } } + if (closedir(cpudir)) + PERROR("closedir"); + + /* + * If the max CPU id is out of bound, set it to -1 so it results in a + * CPU num of 0. + */ + if (max_cpuid < 0 || max_cpuid > INT_MAX) + max_cpuid = -1; + end: /* - * Get the sysconf value as a fallback. Keep the highest number. + * Get the sysconf value as a last resort. Keep the highest number. */ - return __max(sysconf(_SC_NPROCESSORS_CONF), count); + return __max(sysconf(_SC_NPROCESSORS_CONF), max_cpuid + 1); } -#endif /* * Get the CPU possible mask string from sysfs. -- 2.34.1