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bertos/kern/sem.c

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+/**
+ * \file
+ * <!--
+ * This file is part of BeRTOS.
+ *
+ * Bertos is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * As a special exception, you may use this file as part of a free software
+ * library without restriction.  Specifically, if other files instantiate
+ * templates or use macros or inline functions from this file, or you compile
+ * this file and link it with other files to produce an executable, this
+ * file does not by itself cause the resulting executable to be covered by
+ * the GNU General Public License.  This exception does not however
+ * invalidate any other reasons why the executable file might be covered by
+ * the GNU General Public License.
+ *
+ * Copyright 2001, 2004 Develer S.r.l. (http://www.develer.com/)
+ * Copyright 1999, 2000, 2001 Bernie Innocenti <bernie@codewiz.org>
+ * -->
+ *
+ * \brief Semaphore based synchronization services.
+ *
+ * \author Bernie Innocenti <bernie@codewiz.org>
+ */
+
+#include "sem.h"
+#include <cfg/debug.h>
+
+#include <cpu/irq.h> // ASSERT_IRQ_DISABLED()
+
+#include <kern/proc.h>
+#include <kern/proc_p.h>
+#include <kern/signal.h>
+
+INLINE void sem_verify(struct Semaphore *s)
+{
+	(void)s;
+	ASSERT(s);
+	LIST_ASSERT_VALID(&s->wait_queue);
+	ASSERT(s->nest_count >= 0);
+	ASSERT(s->nest_count < 128);   // heuristic max
+}
+
+#if CONFIG_KERN_PRI && CONFIG_KERN_PRI_INHERIT
+
+#define proc_updatePri(proc) (proc_setPri(proc, (proc)->orig_pri))
+
+
+/**
+ * Priority inheritance update algorithm.
+ *
+ * The algorithm checks and boosts the priority of the semaphore's
+ * current owner and also processes in that block the owner, which
+ * form a chain of blocking processes.
+ *
+ * Note that the priority of a process in the chain of blocked
+ * processes is always greater or equal than the priority of a process
+ * before in the chain. See the diagram below:
+ * P1  --. S1 ---> P2 --. S2 ---> P3
+ * prio_proc(P2) >= prio_proc(P1) always.
+ */
+INLINE void pri_inheritBlock(Semaphore *s)
+{
+	Process *owner = s->owner;
+
+	/*
+	 * Enqueue the blocking process in the owner's inheritance
+	 * list. Notice that such process might have inherited its
+	 * current priority from someone else.
+	 */
+	current_process->inh_link.pri = __prio_proc(current_process);
+	LIST_ENQUEUE(&owner->inh_list, &current_process->inh_link);
+	current_process->inh_blocked_by = s;
+
+	/*
+	 * As long as a process has the power of boosting the priority
+	 * of its lock owner...
+	 */
+	while (current_process->inh_link.pri > prio_proc(owner)) {
+		Process *p = owner;
+
+		 /* Boost the priority of the owner */
+		proc_updatePri(p);
+
+		/* If the owner is not blocked, we're done */
+		if (!p->inh_blocked_by)
+			break;
+
+		/*
+		 * Otherwise update the position of the owner
+		 * (which is `p' at each round!) in the inheritance
+		 * list it lies in and set up `owner' for the
+		 * next round.
+		 */
+		REMOVE(&p->inh_link.link);
+		p->inh_link.pri = prio_proc(p);
+		owner = p->inh_blocked_by->owner;
+		LIST_ENQUEUE(&owner->inh_list, &p->inh_link);
+	}
+}
+
+
+/**
+ * Priority inheritance unblock algorithm.
+ *
+ * Pass the priority inheritance list from the current owner to the
+ * process that will take ownership of the semaphore next, potentially
+ * boosting its priority.
+ *
+ * \param proc The process that will take ownership of the semaphore.
+ */
+INLINE void pri_inheritUnblock(Semaphore *s, Process *proc)
+{
+	Process *owner = s->owner;
+	Node *n, *temp;
+	Process *p;
+
+	/*
+	 * This process has nothing more to do on a priority
+	 * inheritance list.
+	 */
+	REMOVE(&proc->inh_link.link);
+	proc->inh_blocked_by = NULL;
+
+	/*
+	 * Each process in the former owner's priority inheritance
+	 * list that is blocked on 's' needs to be removed from
+	 * there and added to the priority inheritance list of
+	 * this process, since it's going to be the new owner for
+	 * that semaphore.
+	 */
+	FOREACH_NODE_SAFE(n, temp, &owner->inh_list) {
+		p = containerof(n, Process, inh_link.link);
+		/* Ensures only the processes blocked on 's' are affected! */
+		if (p->inh_blocked_by == s) {
+			REMOVE(&p->inh_link.link);
+			LIST_ENQUEUE(&proc->inh_list, &p->inh_link);
+
+			/* And again, update the priority of the new owner */
+			if (p->inh_link.pri > prio_proc(proc))
+				proc_updatePri(proc);
+		}
+	}
+
+	proc_updatePri(owner);
+}
+#else
+INLINE void pri_inheritBlock(UNUSED_ARG(Semaphore *, s))
+{
+}
+
+INLINE void pri_inheritUnblock(UNUSED_ARG(Semaphore *, s), UNUSED_ARG(Process *, proc))
+{
+}
+#endif /* CONFIG_KERN_PRI_INHERIT */
+
+
+/**
+ * \brief Initialize a Semaphore structure.
+ */
+void sem_init(struct Semaphore *s)
+{
+	LIST_INIT(&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().
+ *
+ * \see sem_obtain() sem_release()
+ */
+bool sem_attempt(struct Semaphore *s)
+{
+	bool result = false;
+
+	proc_forbid();
+	sem_verify(s);
+	if ((!s->owner) || (s->owner == current_process))
+	{
+		s->owner = current_process;
+		s->nest_count++;
+		result = true;
+	}
+	proc_permit();
+
+	return result;
+}
+
+
+/**
+ * \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.
+ *
+ * \sa sem_release() sem_attempt()
+ */
+void sem_obtain(struct Semaphore *s)
+{
+	proc_forbid();
+	sem_verify(s);
+
+	/* Is the semaphore already locked by another process? */
+	if (UNLIKELY(s->owner && (s->owner != current_process)))
+	{
+		/* Append calling process to the wait queue */
+		ADDTAIL(&s->wait_queue, (Node *)current_process);
+
+		/* Trigger priority inheritance logic, if enabled */
+		pri_inheritBlock(s);
+
+		/*
+		 * We will wake up only when the current owner calls
+		 * sem_release(). Then, the semaphore will already
+		 * be locked for us.
+		 */
+		proc_permit();
+		proc_switch();
+	}
+	else
+	{
+		ASSERT(LIST_EMPTY(&s->wait_queue));
+
+		/* The semaphore was free: lock it */
+		s->owner = current_process;
+		s->nest_count++;
+		proc_permit();
+	}
+}
+
+
+/**
+ * \brief Release 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.
+ *
+ * \sa sem_obtain() sem_attempt()
+ */
+void sem_release(struct Semaphore *s)
+{
+	Process *proc = NULL;
+
+	proc_forbid();
+	sem_verify(s);
+
+	ASSERT(s->owner == current_process);
+
+	/*
+	 * Decrement nesting count and check if the semaphore
+	 * has been fully unlocked.
+	 */
+	if (--s->nest_count == 0)
+	{
+		/* Give semaphore to the first applicant, if any */
+		if (UNLIKELY((proc = (Process *)list_remHead(&s->wait_queue))))
+		{
+			/* Undo the effects of priority inheritance, if enabled */
+			pri_inheritUnblock(s, proc);
+
+			s->nest_count = 1;
+			s->owner = proc;
+		} else {
+			/* Disown semaphore */
+			s->owner = NULL;
+		}
+	}
+	proc_permit();
+
+	if (proc)
+		ATOMIC(proc_wakeup(proc));
+}