Changeset a55d76b1 in mainline for uspace/lib/c/generic/fibril_synch.c

Timestamp:

2018-06-13T17:07:58Z (7 years ago)

Author:

Jiří Zárevúcky <jiri.zarevucky@…>

Branches:

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

Children:

be2a20ac, c407b98

Parents:

899342e

git-author:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-06-13 17:04:01)

git-committer:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-06-13 17:07:58)

Message:

Implement a simple counting semaphore for fibrils.

File:

• uspace/lib/c/generic/fibril_synch.c (modified) (1 diff)

uspace/lib/c/generic/fibril_synch.c

@@ -629 +629 @@
 }
 
+/**
+ * Initialize a semaphore with initial count set to the provided value.
+ *
+ * @param sem    Semaphore to initialize.
+ * @param count  Initial count. Must not be negative.
+ */
+void fibril_semaphore_initialize(fibril_semaphore_t *sem, long count)
+{
+        /*
+         * Negative count denotes the length of waitlist,
+         * so it makes no sense as an initial value.
+         */
+        assert(count >= 0);
+        sem->count = count;
+        list_initialize(&sem->waiters);
+}
+
+/**
+ * Produce one token.
+ * If there are fibrils waiting for tokens, this operation satisfies
+ * exactly one waiting `fibril_semaphore_down()`.
+ * This operation never blocks the fibril.
+ *
+ * @param sem  Semaphore to use.
+ */
+void fibril_semaphore_up(fibril_semaphore_t *sem)
+{
+        futex_down(&async_futex);
+        sem->count++;
+
+        if (sem->count > 0) {
+                futex_up(&async_futex);
+                return;
+        }
+
+        link_t *tmp = list_first(&sem->waiters);
+        assert(tmp);
+        list_remove(tmp);
+
+        futex_up(&async_futex);
+
+        awaiter_t *wdp = list_get_instance(tmp, awaiter_t, wu_event.link);
+        fibril_add_ready(wdp->fid);
+        optimize_execution_power();
+}
+
+/**
+ * Consume one token.
+ * If there are no available tokens (count <= 0), this operation blocks until
+ * another fibril produces a token using `fibril_semaphore_up()`.
+ *
+ * @param sem  Semaphore to use.
+ */
+void fibril_semaphore_down(fibril_semaphore_t *sem)
+{
+        futex_down(&async_futex);
+        sem->count--;
+
+        if (sem->count >= 0) {
+                futex_up(&async_futex);
+                return;
+        }
+
+        awaiter_t wdata;
+        awaiter_initialize(&wdata);
+
+        wdata.fid = fibril_get_id();
+        list_append(&wdata.wu_event.link, &sem->waiters);
+        fibril_switch(FIBRIL_TO_MANAGER);
+
+        /* async_futex not held after fibril_switch() */
+}
+
 /** @}
  */