Fix condition variable broadcasting

The broadcast implementation is based on an invalid assumption, namely
that the first mutex_unlock call following condition_wait would be invoked
on the same mutex. Fixing this while guarding against the thundering herd
effect requires augmenting mutexes with a pointer to the condition
variable they may be associated with. Since the size of mutexes is
currently more important than broadcast scalability, the implementation
is simplified into one which suffers from the thundering herd effect.

1 files changed, 1 insertions, 47 deletions

diff --git a/kern/condition.c b/kern/condition.c
index e6d65951..c407e949 100644
--- a/kern/condition.c
+++ b/kern/condition.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2017 Richard Braun.
+ * Copyright (c) 2013-2018 Richard Braun.
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -27,46 +27,21 @@
 #include <kern/condition_types.h>
 #include <kern/mutex.h>
 #include <kern/sleepq.h>
-#include <kern/thread.h>
 
 static int
 condition_wait_common(struct condition *condition, struct mutex *mutex,
                       bool timed, uint64_t ticks)
 {
-    struct condition *last_cond;
     struct sleepq *sleepq;
     unsigned long flags;
     int error;
 
     mutex_assert_locked(mutex);
 
-    /*
-     * Special case :
-     *
-     * mutex_lock(lock);
-     *
-     * for (;;) {
-     *     while (!done) {
-     *         condition_wait(condition, lock);
-     *     }
-     *
-     *     do_something();
-     * }
-     *
-     * Pull the last condition before unlocking the mutex to prevent
-     * mutex_unlock() from reacquiring the condition sleep queue.
-     */
-    last_cond = thread_pull_last_cond();
-
     sleepq = sleepq_lend(condition, true, &flags);
 
     mutex_unlock(mutex);
 
-    if (last_cond != NULL) {
-        assert(last_cond == condition);
-        sleepq_wakeup(sleepq);
-    }
-
     if (timed) {
         error = sleepq_timedwait(sleepq, "cond", ticks);
     } else {
@@ -74,10 +49,6 @@ condition_wait_common(struct condition *condition, struct mutex *mutex,
         error = 0;
     }
 
-    if (!error) {
-        thread_set_last_cond(condition);
-    }
-
     sleepq_return(sleepq, flags);
 
     mutex_lock(mutex);
@@ -134,20 +105,3 @@ condition_broadcast(struct condition *condition)
 
     sleepq_release(sleepq, flags);
 }
-
-void
-condition_wakeup(struct condition *condition)
-{
-    struct sleepq *sleepq;
-    unsigned long flags;
-
-    sleepq = sleepq_acquire(condition, true, &flags);
-
-    if (sleepq == NULL) {
-        return;
-    }
-
-    sleepq_wakeup(sleepq);
-
-    sleepq_release(sleepq, flags);
-}