Changeset 4b74488 in mainline

Timestamp:

2006-06-09T23:55:56Z (19 years ago)

Author:

Jakub Jermar <jakub@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

6eb96fce

Parents:

7b3e7f4

Message:

Avoid tricky race condition between waitq_wakeup() and the pair
of waitq_timeouted_sleep() and waitq_interrupt_sleep().
Mutual exclusion != Synchronization.

File:

• generic/src/synch/waitq.c (modified) (3 diffs)

generic/src/synch/waitq.c

@@ -100 +100 @@
                 t->saved_context = t->sleep_timeout_context;
                 do_wakeup = true;
-                
+                t->sleep_queue = NULL;
                 spinlock_unlock(&wq->lock);
-                t->sleep_queue = NULL;
         }
         
@@ -155 +154 @@
                 t->saved_context = t->sleep_interruption_context;
                 do_wakeup = true;
-                
+                t->sleep_queue = NULL;
                 spinlock_unlock(&wq->lock);
-                t->sleep_queue = NULL;
         }
         spinlock_unlock(&t->lock);
@@ -419 +417 @@
         t = list_get_instance(wq->head.next, thread_t, wq_link);
         
+        /*
+         * Lock the thread prior to removing it from the wq.
+         * This is not necessary because of mutual exclusion
+         * (the link belongs to the wait queue), but because
+         * of synchronization with waitq_timeouted_sleep()
+         * and waitq_interrupt_sleep().
+         *
+         * In order for these two functions to work, the following
+         * invariant must hold:
+         *
+         * t->sleep_queue != NULL <=> t sleeps in a wait queue
+         *
+         * For an observer who locks the thread, the invariant
+         * holds only when the lock is held prior to removing
+         * it from the wait queue.
+         */
+        spinlock_lock(&t->lock);
         list_remove(&t->wq_link);
-        spinlock_lock(&t->lock);
+        
         if (t->timeout_pending && timeout_unregister(&t->sleep_timeout))
                 t->timeout_pending = false;