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get_nprocs: Only return explictly set cache values (BZ #16996)

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The implementation of __get_nprocs uses a stactic variable to cache
the value of the current number of processors.  The caching breaks when
'time (NULL) == 0':

  $ cat nproc.c
  #include <stdio.h>
  #include <time.h>
  #include <sys/time.h>

  int main(int argc, char *argv[])
  {
    time_t t;
    struct timeval tv = {0, 0};
    printf("settimeofday({0, 0}, NULL) = %d\n", settimeofday(&tv, NULL));
    t = time(NULL);
    printf("Time: %d, CPUs: %d\n", (unsigned int)t, get_nprocs());
    return 0;
  }
  $ gcc -O3 nproc.c
  $ ./a.out
  settimeofday({0, 0}, NULL) = -1
  Time: 1401311578, CPUs: 4
  $ sudo ./a.out
  settimeofday({0, 0}, NULL) = 0
  Time: 0, CPUs: 0

The problem is with the condition used to check whether a cached
value should be returned or not:

  static int cached_result;
  static time_t timestamp;

  time_t now = time (NULL);
  time_t prev = timestamp;
  atomic_read_barrier ();
  if (now == prev)
    return cached_result;

This patch fixes the problem by ensuring that 'cached_result' has
been set at least once before returning it.
This commit is contained in:
Meador Inge
2014-06-13 14:02:04 +05:30
committed by Siddhesh Poyarekar
parent febf6cc58c
commit 995a46bbfb
3 changed files with 9 additions and 3 deletions
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4
sysdeps/unix/sysv/linux/getsysstats.c
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@@ -126,13 +126,13 @@ next_line (int fd, char *const buffer, char **cp, char **re,
int
__get_nprocs (void)
{
static int cached_result;
static int cached_result = -1;
static time_t timestamp;
time_t now = time (NULL);
time_t prev = timestamp;
atomic_read_barrier ();
if (now == prev)
if (now == prev && cached_result > -1)
return cached_result;
/* XXX Here will come a test for the new system call. */