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libbpf : make libbpf_num_possible_cpus function thread safe
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 <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>
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@ -4995,13 +4995,15 @@ int libbpf_num_possible_cpus(void)
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static const char *fcpu = "/sys/devices/system/cpu/possible";
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int len = 0, n = 0, il = 0, ir = 0;
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unsigned int start = 0, end = 0;
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int tmp_cpus = 0;
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static int cpus;
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char buf[128];
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int error = 0;
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int fd = -1;
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if (cpus > 0)
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return cpus;
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tmp_cpus = READ_ONCE(cpus);
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if (tmp_cpus > 0)
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return tmp_cpus;
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fd = open(fcpu, O_RDONLY);
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if (fd < 0) {
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@ -5024,7 +5026,7 @@ int libbpf_num_possible_cpus(void)
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}
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buf[len] = '\0';
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for (ir = 0, cpus = 0; ir <= len; ir++) {
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for (ir = 0, tmp_cpus = 0; ir <= len; ir++) {
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/* Each sub string separated by ',' has format \d+-\d+ or \d+ */
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if (buf[ir] == ',' || buf[ir] == '\0') {
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buf[ir] = '\0';
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@ -5036,13 +5038,15 @@ int libbpf_num_possible_cpus(void)
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} else if (n == 1) {
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end = start;
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}
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cpus += end - start + 1;
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tmp_cpus += end - start + 1;
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il = ir + 1;
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}
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}
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if (cpus <= 0) {
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pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
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if (tmp_cpus <= 0) {
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pr_warning("Invalid #CPUs %d from %s\n", tmp_cpus, fcpu);
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return -EINVAL;
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}
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return cpus;
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WRITE_ONCE(cpus, tmp_cpus);
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return tmp_cpus;
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}
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