[Python-checkins] cpython: Issue #15038:

Tue Jun 19 12:11:11 CEST 2012

http://hg.python.org/cpython/rev/110b38c36a31
changeset: 77515:110b38c36a31
user: Kristjan Valur Jonsson <sweskman at gmail.com>
date: Tue Jun 19 10:10:09 2012 +0000
summary:
 Issue #15038:
Fix incorrect test of the condition variable state, spotted by
Richard Oudkerk. This could cause the internal condition variable
to grow without bounds.
files:
 Python/condvar.h | 18 +++++++++++++-----
 1 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/Python/condvar.h b/Python/condvar.h
--- a/Python/condvar.h
+++ b/Python/condvar.h
@@ -177,7 +177,7 @@
 typedef struct _PyCOND_T
 {
 HANDLE sem;
- int waiting;
+ int waiting; /* to allow PyCOND_SIGNAL to be a no-op */
 } PyCOND_T;
 
 Py_LOCAL_INLINE(int)
@@ -222,6 +222,10 @@
 * PyCOND_SIGNAL also decrements this value
 * and signals releases the mutex. This is benign because it
 * just means an extra spurious wakeup for a waiting thread.
+ * ('waiting' corresponds to the semaphore's "negative" count and
+ * we may end up with e.g. (waiting == -1 && sem.count == 1). When
+ * a new thread comes along, it will pass right throuhgh, having
+ * adjusted it to (waiting == 0 && sem.count == 0).
 */
 
 if (wait == WAIT_FAILED)
@@ -246,10 +250,14 @@
 Py_LOCAL_INLINE(int)
 PyCOND_SIGNAL(PyCOND_T *cv)
 {
- if (cv->waiting) {
+ /* this test allows PyCOND_SIGNAL to be a no-op unless required
+ * to wake someone up, thus preventing an unbounded increase of
+ * the semaphore's internal counter.
+ */
+ if (cv->waiting > 0) {
 /* notifying thread decreases the cv->waiting count so that
- * a delay between notify and wakeup doesn't cause a number
- * of extra ReleaseSemaphore calls
+ * a delay between notify and actual wakeup of the target thread
+ * doesn't cause a number of extra ReleaseSemaphore calls.
 */
 cv->waiting--;
 return ReleaseSemaphore(cv->sem, 1, NULL) ? 0 : -1;
@@ -260,7 +268,7 @@
 Py_LOCAL_INLINE(int)
 PyCOND_BROADCAST(PyCOND_T *cv)
 {
- if (cv->waiting) {
+ if (cv->waiting > 0) {
 return ReleaseSemaphore(cv->sem, cv->waiting, NULL) ? 0 : -1;
 		cv->waiting = 0;
 }
-- 
Repository URL: http://hg.python.org/cpython
