test: fix excessive loom branches with Backoff (#472)

This fixes an issue where the `util::Backoff` helper in `maitake_sync`
would emit too many spins in loom mode, hitting the branch limit.

maitake-sync/src/util.rs

@@ -9,14 +9,6 @@
 //!
 //! - [`Backoff`]: exponential backoff for spin loops
 //! - [`CachePadded`]: pads and aligns a value to the size of a cache line
-mod backoff;
-mod cache_pad;
-pub(crate) mod fmt;
-mod maybe_uninit;
-mod wake_batch;
-
-pub use self::{backoff::Backoff, cache_pad::CachePadded};
-pub(crate) use self::{maybe_uninit::CheckedMaybeUninit, wake_batch::WakeBatch};
 
 #[cfg(any(test, feature = "tracing"))]
 macro_rules! trace {
@@ -196,8 +188,16 @@ macro_rules! unreachable_unchecked {
     });
 }
 
+mod backoff;
+mod cache_pad;
+pub(crate) mod fmt;
+mod maybe_uninit;
+mod wake_batch;
+
 #[cfg(all(test, not(loom)))]
 pub(crate) use self::test::trace_init;
+pub use self::{backoff::Backoff, cache_pad::CachePadded};
+pub(crate) use self::{maybe_uninit::CheckedMaybeUninit, wake_batch::WakeBatch};
 
 #[cfg(test)]
 pub(crate) mod test {

maitake-sync/src/util/backoff.rs

@@ -1,5 +1,3 @@
-use crate::loom::hint;
-
 /// An [exponential backoff] for spin loops.
 ///
 /// This is a helper struct for spinning in a busy loop, with an exponentially
@@ -53,8 +51,16 @@ impl Backoff {
     #[inline(always)]
     pub fn spin(&mut self) {
         // Issue 2^exp pause instructions.
-        for _ in 0..1 << self.exp {
-            hint::spin_loop();
+        let spins = 1 << self.exp;
+        #[cfg(not(loom))]
+        for _ in 0..spins {
+            crate::loom::hint::spin_loop();
+        }
+
+        #[cfg(loom)]
+        {
+            test_debug!("would back off for {spins} spins");
+            loom::thread::yield_now();
         }
 
         if self.exp < self.max {