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libbpf : make libbpf_num_possible_cpus function thread safe

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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: 6446b31555 ("bpf: add a new API libbpf_num_possible_cpus()")
Signed-off-by: Takshak Chahande <ctakshak@fb.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Takshak Chahande 2019-07-31 15:10:55 -07:00 committed by Alexei Starovoitov
parent 5d01ab7bac
commit 56fbc24116
1 changed files with 11 additions and 7 deletions
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18
tools/lib/bpf/libbpf.c
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@ -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;
}