Support conditional SystemTap probes using semaphores

[nbaksalyar]

This is a proof-of-concept implementation of #16 (more of an RFC for now rather than a complete thing).

Unfortunately, this implementation requires use of procedural macros because there's no other easy way to define static variables in a macro (global vars defined in inline asm didn't work).

I also couldn't find any easy way to implement probe_enabled!. Most likely, it will require the use of something like std::sync::Once/lazy_static to maintain a "registry" of probes. Even then, SystemTap expects a link-time address of a semaphore variable (unless we want to use dynamically defined probes?). I think having a separate macro like probe_opt! which will lazily evaluate its arguments is a fine compromise.

• Use a separate probe_opt! macro for probes with semaphores.
• Document the semaphores implementation.
• Document procedural macros and naming conventions.
• Add more examples using semaphores.
• Add tests.
• Add usage examples in readme.

[cuviper]

If we set aside probe_enabled!, I think #![feature(asm_sym)] (rust-lang/rust#93333) will make this much easier. We can let macro hygiene deal with creating a unique mangled name for the semaphore, and just pass that to the assembly.

--- a/src/platform/systemtap.rs
+++ b/src/platform/systemtap.rs
@@ -101,6 +101,8 @@ macro_rules! sdt_asm(
 #[macro_export]
 macro_rules! _sdt_asm(
     ($size:literal, options ($($opt:ident),*), $provider:ident, $name:ident, $($argstr:literal, $arg:expr,)*) => (
+        static mut SEMAPHORE: u16 = 0;
+        if SEMAPHORE > 0 {
         ::core::arch::asm!(concat!(r#"
 990:    nop
         .pushsection .note.stapsdt,"?","note"
@@ -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#""
@@ -126,7 +128,9 @@ _.stapsdt.base: .space 1
 .endif
 "#
             ),
+            sym SEMAPHORE,
             $(in(reg) (($arg) as isize) ,)*
             options(readonly, nostack, preserves_flags, $($opt),*),
         )
+        }
     ));

This works for me for lazy evaluation, even in a release build. I suppose we could add a #[link_section = ".probes"] on that static, but I honestly don't remember why SystemTap does that.

What do you think?

[nbaksalyar]

feature(asm_sym)

I wasn’t aware of this, looks awesome! Definitely makes it a lot simpler. But it will require to wait for the next stable release or to use the nightly toolchain though, right?

I think the .probes section is optional (but I’ll double-check that).

[cuviper]

But it will require to wait for the next stable release or to use the nightly toolchain though, right?

It would be in the next full release cycle after they actually stabilize that feature, yes, but it looks like that might be close. e.g. that would be headed for Rust 1.62 if it stabilized today. In the meantime, it needs nightly with the opt-in feature attribute, but I'd rather wait for stabilization.

[nbaksalyar]

I'd rather wait for stabilization.

Cool, I’m going to fix this PR and add docs/examples in the meantime then.

[cuviper]

FYI, asm_sym is stabilizing in Rust 1.66 tomorrow, December 15th.

[cuviper]

Thanks! I've published 0.4.0.

[nbaksalyar]

Thanks! I've published 0.4.0.

Excellent, thank you so much for reviewing and the improvements :)