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authorTakshak Chahande <ctakshak@fb.com>2019-07-31 15:10:55 -0700
committerAlexei Starovoitov <ast@kernel.org>2019-07-31 20:35:19 -0700
commit56fbc24116f458a0ea48f9f37fe770fd791042d9 (patch)
tree17d137a6c297c88c4ad99dddf76b382ab9db9b42 /tools/lib
parent5d01ab7bac467edfc530e6ccf953921def935c62 (diff)
downloadlinux-56fbc24116f458a0ea48f9f37fe770fd791042d9.tar.bz2
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>
Diffstat (limited to 'tools/lib')
-rw-r--r--tools/lib/bpf/libbpf.c18
1 files 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;
}