Import kern/ subdirectory.

[bertos.git] / kern / sem.c

diff --git a/kern/sem.c b/kern/sem.c
new file mode 100755 (executable)
index 0000000..98646e9
--- /dev/null
+++ b/kern/sem.c
@@ -0,0 +1,144 @@
+/*!
+ * \file
+ * <!--
+ * Copyright 2001,2004 Develer S.r.l. (http://www.develer.com/)
+ * Copyright 1999,2000,2001 Bernardo Innocenti <bernie@develer.com>
+ * All Rights Reserved.
+ * -->
+ *
+ * \brief Semaphore based synchronization services.
+ *
+ * \version $Id$
+ *
+ * \author Bernardo Innocenti <bernie@develer.com>
+ */
+
+/*
+ * $Log$
+ * Revision 1.1  2004/05/23 17:27:00  bernie
+ * Import kern/ subdirectory.
+ *
+ */
+
+#include "sem.h"
+#include "proc.h"
+#include "proc_p.h"
+#include "signal.h"
+#include "hw.h"
+
+
+/*!
+ * \brief Initialize a Semaphore structure
+ */
+void sem_init(struct Semaphore *s)
+{
+       INITLIST(&s->wait_queue);
+       s->owner = NULL;
+       s->nest_count = 0;
+}
+
+
+/*!
+ * \brief Attempt to lock a semaphore without waiting.
+ *
+ * \return true in case of success, false if the semaphore
+ *         was already locked by someone else.
+ *
+ * \note   each call to sem_attempt() must be matched by a
+ *         call to sem_release().
+ */
+bool sem_attempt(struct Semaphore *s)
+{
+       DISABLE_INTS;
+       if ((!s->owner) || (s->owner == CurrentProcess))
+       {
+               s->owner = CurrentProcess;
+               s->nest_count++;
+               ENABLE_INTS;
+               return true;
+       }
+       ENABLE_INTS;
+       return false;
+}
+
+
+/*!
+ * \brief Lock a semaphore.
+ *
+ * If the semaphore is already owned by another process, the caller
+ * process will be enqueued into the waiting list and sleep until
+ * the semaphore is available.
+ *
+ * \note Each call to sem_obtain() must be matched by a
+ *       call to sem_release().
+ *
+ * \note This routine is optimized for highest speed in
+ *       the most common case: the semaphore is free or locked
+ *       by the calling process itself. Rearranging this code
+ *       is probably a bad idea.
+ */
+void sem_obtain(struct Semaphore *s)
+{
+       DISABLE_INTS;
+
+       /* Is the semaphore already locked by another process? */
+       if (s->owner && (s->owner != CurrentProcess))
+       {
+               /* Append calling process to the wait queue */
+               ADDTAIL(&s->wait_queue, (Node *)CurrentProcess);
+               ENABLE_INTS;
+
+               /* We will awake only when the current owner calls
+                * ReleaseSemaphore(). Then, the semaphore will already
+                * be locked for us.
+                */
+               proc_schedule();
+       }
+       else
+       {
+               /* The semaphore is free: lock it */
+               s->owner = CurrentProcess;
+               s->nest_count++;
+               ENABLE_INTS;
+       }
+}
+
+
+/*!
+ * \brief Releases a lock on a previously locked semaphore.
+ *
+ * If the nesting count of the semaphore reaches zero,
+ * the next process waiting for it will be awaken.
+ *
+ * \note This routine is optimized for highest speed in
+ *       the most common case: the semaphore has been locked just
+ *       once and nobody else was waiting for it. Rearranging
+ *       this code is probably a bad idea.
+ */
+void sem_release(struct Semaphore *s)
+{
+       DISABLE_INTS;
+
+       /* Decremement nesting count and check if the semaphore
+        * has been fully unlocked
+        */
+       if (--s->nest_count == 0)
+       {
+               /* Disown semaphore */
+               s->owner = NULL;
+
+               /* Anybody still waiting for this semaphore? */
+               if (!ISLISTEMPTY(&s->wait_queue))
+               {
+                       /* Give semaphore to the first applicant */
+                       Process *proc = (Process *)s->wait_queue.head;
+                       REMOVE((Node *)proc);
+                       s->nest_count = 1;
+                       s->owner = proc;
+                       SCHED_ENQUEUE(proc);
+               }
+       }
+
+       ENABLE_INTS;
+}
+