libbpf : make libbpf_num_possible_cpus function thread safe (56fbc241) · Commits · 戴 / test

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: ("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>

tools/lib/bpf/libbpf.c

+11 −7

Original line number	Diff line number	Diff line
		@@ -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;
		}

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