spl.c

/*
 * Copyright (c) 2009
 *      The President and Fellows of Harvard College.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/* Make sure to build out-of-line versions of spl inline functions */
#define SPL_INLINE      /* empty */

#include <types.h>
#include <lib.h>
#include <cpu.h>
#include <spl.h>
#include <thread.h>
#include <current.h>

/*
 * Machine-independent interrupt handling functions.
 *
 * Traditionally, all this code is machine-dependent.
 *
 * However.
 *
 * Since on OS/161 we don't support interrupt levels on any platform,
 * all we require under this logic is cpu_irqoff() and cpu_irqon()
 * that explicitly turn interrupts off and on.
 *
 * If we had multiple interrupt levels, the number of levels would in
 * general be different on different platforms (depending on hardware
 * requirements and hardware capabilities) so things would get more
 * complicated -- but nearly all of this code could remain MI.
 */


/*
 * Raise and lower the interrupt priority level.
 *
 * Each spinlock acquisition can raise and lower the priority level
 * independently. The spl calls also raise and lower the priority
 * level independently of the spinlocks. This is necessary because in
 * general spinlock acquisitions and releases don't nest perfectly,
 * and don't necessarily nest with respect to spl calls either.
 *
 * For example:
 *
 *    struct spinlock red, blue;
 *    int s;
 *
 *    spinlock_acquire(&red);
 *    s = splhigh();
 *    spinlock_acquire(&blue);
 *    splx(s);
 *    spinlock_release(&red);
 *    spinlock_release(&blue);
 *
 * In order to make this work we need to count the number of times
 * IPL_HIGH (or, if we had multiple interrupt priority levels, each
 * level independently) has been raised. Interrupts go off on the
 * first raise, and go on again only on the last lower.
 *
 * curthread->t_iplhigh_count is used to track this.
 */
void
splraise(int oldspl, int newspl)
{
        struct thread *cur = curthread;

        /* only one priority level, only one valid args configuration */
        KASSERT(oldspl == IPL_NONE);
        KASSERT(newspl == IPL_HIGH);

        if (!CURCPU_EXISTS()) {
                /* before curcpu initialization; interrupts are off anyway */
                return;
        }

        if (cur->t_iplhigh_count == 0) {
                cpu_irqoff();
        }
        cur->t_iplhigh_count++;
}

void
spllower(int oldspl, int newspl)
{
        struct thread *cur = curthread;

        /* only one priority level, only one valid args configuration */
        KASSERT(oldspl == IPL_HIGH);
        KASSERT(newspl == IPL_NONE);

        if (!CURCPU_EXISTS()) {
                /* before curcpu initialization; interrupts are off anyway */
                return;
        }

        cur->t_iplhigh_count--;
        if (cur->t_iplhigh_count == 0) {
                cpu_irqon();
        }
}


/*
 * Disable or enable interrupts and adjust curspl setting. Return old
 * spl level.
 */
int
splx(int spl)
{
        struct thread *cur = curthread;
        int ret;

        if (cur->t_curspl < spl) {
                /* turning interrupts off */
                splraise(cur->t_curspl, spl);
                ret = cur->t_curspl;
                cur->t_curspl = spl;
        }
        else if (cur->t_curspl > spl) {
                /* turning interrupts on */
                ret = cur->t_curspl;
                cur->t_curspl = spl;
                spllower(ret, spl);
        }
        else {
                /* do nothing */
                ret = spl;
        }

        return ret;
}