X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2Ftour.texi;h=d9dab057a06f88dab97099d32ef16f98badcf2fb;hb=9fd80c814b9b3f95c16a90a680250c3fd6e5b74e;hp=ed1f92be1bad527cdc7574371dc917a16f7b603b;hpb=36771800f7bfa985736db684d0c820c96db887ae;p=pintos-anon

diff --git a/doc/tour.texi b/doc/tour.texi
index ed1f92b..d9dab05 100644
--- a/doc/tour.texi
+++ b/doc/tour.texi
@@ -553,8 +553,10 @@ A semaphore initialized to 0 can be useful for waiting for an event
 that will happen exactly once.  For example, suppose thread @var{A}
 starts another thread @var{B} and wants to wait for @var{B} to signal
 that some activity is complete.  @var{A} can create a semaphore
-initialized to 0, pass it to @var{B}, and then ``down'' the semaphore.
-When @var{B} finishes its activity, it ``ups'' the semaphore.
+initialized to 0, pass it to @var{B} as it starts it, and then
+``down'' the semaphore.  When @var{B} finishes its activity, it
+``ups'' the semaphore.  This works regardless of whether @var{A}
+``downs'' the semaphore or @var{B} ``ups'' it first.
 
 Pintos declared its semaphore type and operations on them in
 @file{threads/synch.h}.
@@ -665,22 +667,22 @@ struct condition not_full; /* @r{Signaled when the buffer is not full.} */
 
 void put (char ch) @{
   lock_acquire (&lock);
-  while (n == BUF_SIZE)         /* @r{Can't add to @var{buf} as long as it's full.} */
-    cond_wait (&not_full);
-  buf[head++ % BUF_SIZE] = ch;  /* @r{Add @var{ch} to @var{buf}.} */
+  while (n == BUF_SIZE)            /* @r{Can't add to @var{buf} as long as it's full.} */
+    cond_wait (&not_full, &lock);
+  buf[head++ % BUF_SIZE] = ch;     /* @r{Add @var{ch} to @var{buf}.} */
   n++;
-  cond_signal (&not_empty);     /* @r{@var{buf} can't be empty anymore.} */
+  cond_signal (&not_empty, &lock); /* @r{@var{buf} can't be empty anymore.} */
   lock_release (&lock);
 @}
 
 char get (void) @{
   char ch;
   lock_acquire (&lock);
-  while (n == 0)                /* @r{Can't read from @var{buf} as long as it's empty.} */
-    cond_wait (&not_empty);
-  ch = buf[tail++ % BUF_SIZE];  /* @r{Get @var{ch} from @var{buf}.} */
+  while (n == 0)                  /* @r{Can't read from @var{buf} as long as it's empty.} */
+    cond_wait (&not_empty, &lock);
+  ch = buf[tail++ % BUF_SIZE];    /* @r{Get @var{ch} from @var{buf}.} */
   n--;
-  cond_signal (&not_full);      /* @r{@var{buf} can't be full anymore.} */
+  cond_signal (&not_full, &lock); /* @r{@var{buf} can't be full anymore.} */
   lock_release (&lock);
 @}
 @end example