restore non-freebsd-unix fix for profiling

src/signals-unix.c

@@ -310,14 +310,34 @@ int exc_reg_is_write_fault(uintptr_t esr) {
 #include <sys/eventfd.h>
 #include <link.h>
 
+#ifndef _OS_FREEBSD_
+typedef struct {
+    void (*f)(void*) JL_NOTSAFEPOINT;
+    void *ctx;
+} callback_t;
+static int with_dl_iterate_phdr_lock(struct dl_phdr_info *info, size_t size, void *data)
+{
+    jl_lock_profile();
+    callback_t *callback = (callback_t*)data;
+    callback->f(callback->ctx);
+    jl_unlock_profile();
+    return 1; // only call this once
+}
+#endif
+
 void jl_with_stackwalk_lock(void (*f)(void*), void *ctx)
 {
-    sigset_t sset, oset;
-    sigemptyset(&sset);
-    sigaddset(&sset, SIGUSR2);
-    pthread_sigmask(SIG_BLOCK, &sset, &oset);
+#ifndef _OS_FREEBSD_
+    callback_t callback = {f, ctx};
+    dl_iterate_phdr(with_dl_iterate_phdr_lock, &callback);
+#else
+    // FreeBSD makes the questionable decisions to use a terrible implementation of a spin
+    // lock and to block all signals while a lock is held. However, that also means it is
+    // not currently vulnerable to this libunwind bug that other platforms can encounter.
+    jl_lock_profile();
     f(ctx);
-    pthread_sigmask(SIG_SETMASK, &oset, NULL);
+    jl_unlock_profile();
+#endif
 }
 
 #if defined(_OS_LINUX_) && (defined(_CPU_X86_64_) || defined(_CPU_X86_))