Takshak Chahande <ctaks...@fb.com> [Wed, 2019-07-31 15:11 -0700]:
> 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
This "Changes" section should be after "---" line not to be included in
the final commit message.
Not sure if resubmit is needed because of it, but other than this looks
good to me.
Acked-by: Andrey Ignatov <r...@fb.com>
> 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
>
--
Andrey Ignatov
