added note regarding console lock

diff --git a/doc/threads.texi b/doc/threads.texi
index 8dad172b4540dffb30722957c7c29a69f02d7698..b13b5450ff00802e06fa346894a6955ece891f4e 100644 (file)
--- a/doc/threads.texi
+++ b/doc/threads.texi
@@ -787,11 +787,19 @@ What is happening is that output from two threads is being
 interleaved.  That is, one thread is printing @code{"(alarm-priority)
 Thread priority 29 woke up.\n"} and another thread is printing
 @code{"(alarm-priority) Thread priority 30 woke up.\n"}, but the first
-thread is being interrupted by the second in the middle of its output.
+thread is being preempted by the second in the middle of its output.
 
 This problem indicates a bug in your priority scheduler.  After all, a
 thread with priority 29 should not be able to run while a thread with
 priority 30 has work to do.
+
+Normally, the implementation of the @code{printf()} function in the
+Pintos kernel attempts to prevent such interleaved output by acquiring
+a console lock during the duration of the @code{printf} call and
+releasing it afterwards.  However, the output of the test name,
+e.g., @code{(alarm-priority)}, and the message following it is output
+using two calls to @code{printf}, resulting in the console lock being
+acquired and released twice.
 @end table
 
 @node Advanced Scheduler FAQ