PoC for SystemTap semaphores - using asm_sym

examples/loop.rs

@@ -1,11 +1,16 @@
+#![feature(asm_sym)]
+
 use probe::probe;
+
 fn main() {
-    probe!(foo, begin);
-    let mut total = 0;
-    for i in 0..100 {
-        total += i;
-        probe!(foo, loop, i, total);
+    let mut iter = 0;
+    loop {
+        iter += 1;
+        probe!(foo, iter, {
+            std::thread::sleep(std::time::Duration::from_secs(1));
+            iter
+        });
     }
-    assert_eq!(total, 4950);
-    probe!(foo, end);
 }
+
+// bcc/tools/trace.py -p $(pidof loop) 'u::foo:iter "iter = %d", arg1'

src/lib.rs

@@ -79,6 +79,7 @@
 //! ```
 
 #![no_std]
+#![feature(asm_sym)]
 
 mod platform;
 

src/platform/systemtap.rs

@@ -101,7 +101,9 @@ macro_rules! sdt_asm(
 #[macro_export]
 macro_rules! _sdt_asm(
     ($size:literal, options ($($opt:ident),*), $provider:ident, $name:ident, $($argstr:literal, $arg:expr,)*) => (
-        ::core::arch::asm!(concat!(r#"
+        static SEMAPHORE: u16 = 0;
+        if ::core::ptr::read_volatile(&SEMAPHORE) == 1 {
+            ::core::arch::asm!(concat!(r#"
 990:    nop
         .pushsection .note.stapsdt,"?","note"
         .balign 4
@@ -110,7 +112,7 @@ macro_rules! _sdt_asm(
 992:    .balign 4
 993:    ."#, $size, r#"byte 990b
         ."#, $size, r#"byte _.stapsdt.base
-        ."#, $size, r#"byte 0 // FIXME set semaphore address
+        ."#, $size, r#"byte {}
         .asciz ""#, stringify!($provider), r#""
         .asciz ""#, stringify!($name), r#""
         .asciz ""#, $($argstr,)* r#""
@@ -123,10 +125,10 @@ macro_rules! _sdt_asm(
 _.stapsdt.base: .space 1
         .size _.stapsdt.base, 1
         .popsection
-.endif
-"#
-            ),
+.endif"#),
+            sym SEMAPHORE,
             $(in(reg) (($arg) as isize) ,)*
             options(readonly, nostack, preserves_flags, $($opt),*),
-        )
+        );
+        }
     ));