[PATCH bpf-next] libbpf: synchronize access to print function pointer

[JP Kobryn <inwardvessel@xxxxxxxxx>]

This patch prevents races on the print function pointer, allowing the
libbpf_set_print() function to become thread safe.

Signed-off-by: JP Kobryn <inwardvessel@xxxxxxxxx>
---
 tools/lib/bpf/libbpf.c | 9 ++++++---
 tools/lib/bpf/libbpf.h | 3 +++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index f6a071db5c6e..15737d7b5a28 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -216,9 +216,10 @@ static libbpf_print_fn_t __libbpf_pr = __base_pr;
 
 libbpf_print_fn_t libbpf_set_print(libbpf_print_fn_t fn)
 {
-	libbpf_print_fn_t old_print_fn = __libbpf_pr;
+	libbpf_print_fn_t old_print_fn;
+
+	old_print_fn = __atomic_exchange_n(&__libbpf_pr, fn, __ATOMIC_RELAXED);
 
-	__libbpf_pr = fn;
 	return old_print_fn;
 }
 
@@ -227,8 +228,10 @@ void libbpf_print(enum libbpf_print_level level, const char *format, ...)
 {
 	va_list args;
 	int old_errno;
+	libbpf_print_fn_t print_fn;
 
-	if (!__libbpf_pr)
+	print_fn = __atomic_load_n(&__libbpf_pr, __ATOMIC_RELAXED);
+	if (!print_fn)
 		return;
 
 	old_errno = errno;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 1615e55e2e79..4478809ff9ca 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -99,6 +99,9 @@ typedef int (*libbpf_print_fn_t)(enum libbpf_print_level level,
 /**
  * @brief **libbpf_set_print()** sets user-provided log callback function to
  * be used for libbpf warnings and informational messages.
+ *
+ * This function is thread safe.
+ *
  * @param fn The log print function. If NULL, libbpf won't print anything.
  * @return Pointer to old print function.
  */
-- 
2.39.2