Avoid polling for no reason (#2551)

Author: bugadani

esp-hal-embassy/src/executor/thread.rs

@@ -124,8 +124,6 @@ This will use software-interrupt 3 which isn't available for anything else to wa
         // we do not care about race conditions between the load and store operations,
         // interrupts will only set this value to true.
         if SIGNAL_WORK_THREAD_MODE[cpu].load(Ordering::SeqCst) {
-            SIGNAL_WORK_THREAD_MODE[cpu].store(false, Ordering::SeqCst);
-
             // if there is work to do, exit critical section and loop back to polling
             unsafe {
                 core::arch::asm!(
@@ -141,6 +139,8 @@ This will use software-interrupt 3 which isn't available for anything else to wa
             // `waiti` if it needs to be inside the CS.
             unsafe { core::arch::asm!("waiti 0") };
         }
+        // If this races and some waker sets the signal, we'll reset it, but still poll.
+        SIGNAL_WORK_THREAD_MODE[cpu].store(false, Ordering::SeqCst);
     }
 
     #[cfg(all(riscv, low_power_wait))]
@@ -149,17 +149,14 @@ This will use software-interrupt 3 which isn't available for anything else to wa
         // interrupts will only set this value to true.
         critical_section::with(|_| {
             // if there is work to do, loop back to polling
-            // TODO can we relax this?
-            if SIGNAL_WORK_THREAD_MODE[cpu].load(Ordering::SeqCst) {
-                SIGNAL_WORK_THREAD_MODE[cpu].store(false, Ordering::SeqCst);
-            }
-            // if not, wait for interrupt
-            else {
+            if !SIGNAL_WORK_THREAD_MODE[cpu].load(Ordering::SeqCst) {
+                // if not, wait for interrupt
                 unsafe { core::arch::asm!("wfi") };
             }
         });
-        // if an interrupt occurred while waiting, it will be serviced
-        // here
+        // if an interrupt occurred while waiting, it will be serviced here
+        // If this races and some waker sets the signal, we'll reset it, but still poll.
+        SIGNAL_WORK_THREAD_MODE[cpu].store(false, Ordering::SeqCst);
     }
 }