From 56fbc24116f458a0ea48f9f37fe770fd791042d9 Mon Sep 17 00:00:00 2001 From: Takshak Chahande Date: Wed, 31 Jul 2019 15:10:55 -0700 Subject: 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 Acked-by: Andrey Ignatov Reviewed-by: Jakub Kicinski Signed-off-by: Alexei Starovoitov --- tools/lib/bpf/libbpf.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) (limited to 'tools') 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; } -- cgit v1.2.3-55-g7522