[PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe

[Takshak Chahande]

Having static variable `cpus` in libbpf_num_possible_cpus function
without guarding it with mutex makes this function thread-unsafe.

If multiple threads accessing this function, in the current form; it
leads to incrementing the static variable value `cpus` in the multiple
of total available CPUs.

Used local stack variable to calculate the number of possible CPUs and
then updated the static variable using WRITE_ONCE().

Changes since v1:
 * added stack variable to calculate cpus
 * serialized static variable update using WRITE_ONCE()
 * fixed Fixes tag

Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
Signed-off-by: Takshak Chahande <ctaks...@fb.com>
---
 tools/lib/bpf/libbpf.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6718d0b90130..2e84fa5b8479 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4995,13 +4995,15 @@ int libbpf_num_possible_cpus(void)
        static const char *fcpu = "/sys/devices/system/cpu/possible";
        int len = 0, n = 0, il = 0, ir = 0;
        unsigned int start = 0, end = 0;
+       int tmp_cpus = 0;
        static int cpus;
        char buf[128];
        int error = 0;
        int fd = -1;
 
-       if (cpus > 0)
-               return cpus;
+       tmp_cpus = READ_ONCE(cpus);
+       if (tmp_cpus > 0)
+               return tmp_cpus;
 
        fd = open(fcpu, O_RDONLY);
        if (fd < 0) {
@@ -5024,7 +5026,7 @@ int libbpf_num_possible_cpus(void)
        }
        buf[len] = '\0';
 
-       for (ir = 0, cpus = 0; ir <= len; ir++) {
+       for (ir = 0, tmp_cpus = 0; ir <= len; ir++) {
                /* Each sub string separated by ',' has format \d+-\d+ or \d+ */
                if (buf[ir] == ',' || buf[ir] == '\0') {
                        buf[ir] = '\0';
@@ -5036,13 +5038,15 @@ int libbpf_num_possible_cpus(void)
                        } else if (n == 1) {
                                end = start;
                        }
-                       cpus += end - start + 1;
+                       tmp_cpus += end - start + 1;
                        il = ir + 1;
                }
        }
-       if (cpus <= 0) {
-               pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
+       if (tmp_cpus <= 0) {
+               pr_warning("Invalid #CPUs %d from %s\n", tmp_cpus, fcpu);
                return -EINVAL;
        }
-       return cpus;
+
+       WRITE_ONCE(cpus, tmp_cpus);
+       return tmp_cpus;
 }
-- 
2.17.1