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 <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/libringbuffer/smp.c b/libringbuffer/smp.c
index b6c2962b9711b0979beeea5f2bfcc0f9dff14272..a507df329ebe80d5d01c42517b8452e95d5fcd1d 100644 (file)
--- a/libringbuffer/smp.c
+++ b/libringbuffer/smp.c
@@ -23,55 +23,47 @@
 #define _LGPL_SOURCE
 #include <assert.h>
 #include <ctype.h>
+#include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
 #include <pthread.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <string.h>
+#include <sys/types.h>
 
 #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 <dirent.h>
-#include <limits.h>
-#include <stdlib.h>
-#include <string.h>
-#include <sys/types.h>
-
-#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.