In idle(), "sti; hlt" must be atomic or we can waste time waiting for

the next clock tick.
Fix inspired by Linus Torvalds in message
<Pine.LNX.4.58.0504071000450.28951@ppc970.osdl.org>.

diff --git a/src/threads/thread.c b/src/threads/thread.c
index 6eda28032b397f109cba706769a12a97788e9302..02f3c7b62bb1f949a3a739be3bad3367591fb33c 100644 (file)
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -299,11 +299,19 @@ idle (void *aux UNUSED)
       /* Let someone else run. */
       intr_disable ();
       thread_block ();
-      intr_enable ();
 
-      /* Use CPU `hlt' instruction to wait for interrupt.
-         See [IA32-v2a] "HLT" and [IA32-v3] 7.7. */
-      asm ("hlt");
+      /* Re-enable interrupts and wait for the next one.
+
+         The `sti' instruction disables interrupts until the
+         completion of the next instruction, so these two
+         instructions are executed atomically.  This atomicity is
+         important; otherwise, an interrupt could be handled
+         between re-enabling interrupts and waiting for the next
+         one to occur, wasting as much as one clock tick worth of
+         time.
+
+         See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3] 7.7. */
+      asm ("sti; hlt");
     }
 }