cpu: enforce a memory barrier inside cpu_relax()

Put a memory barrier inside cpu_relax() to make sure the compiler
doesn't cache variables used outside cpu_relax() in registers.

Suppose to have the following case (used in many device drivers):

static int done = false;

void irq_handler(void)
{
	done = true;
}

void wait_for_completion(void)
{
	while (done == false)
		cpu_relax();
}

If cpu_relax() is just considered a nop the compiler may decide to not
reload the value in the variable "done" from memory, causing a
neverending loop.

The presence of an explicit memory barrier fixes this case.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4344 38d2e660-2303-0410-9eaa-f027e97ec537

diff --git a/bertos/cpu/power.h b/bertos/cpu/power.h
index 4e88def111bf414471cdd43adb66aefb76b5d958..f161c698f40ebf14f261d4b77323138e45f98cfb 100644 (file)
--- a/bertos/cpu/power.h
+++ b/bertos/cpu/power.h
@@ -68,6 +68,7 @@
  */
 INLINE void cpu_relax(void)
 {
+       MEMORY_BARRIER;
 #if CONFIG_KERN
        if (proc_preemptAllowed())
                proc_yield();