[bertos.git] / bertos / kern / signal.c

/**
 * \file
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 * library without restriction.  Specifically, if other files instantiate
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 * Copyright 2004, 2008 Develer S.r.l. (http://www.develer.com/)
 * Copyright 1999, 2000, 2001 Bernie Innocenti <bernie@codewiz.org>
 * -->
 *
 * \brief IPC signals implementation.
 *
 * Signals are a low-level IPC primitive.  A process receives a signal
 * when some external event has happened.  Like interrupt requests,
 * signals do not carry any additional information.  If processing a
 * specific event requires additional data, the process must obtain it
 * through some other mechanism.
 *
 * Despite the name, one shouldn't confuse these signals with POSIX
 * signals.  POSIX signals are usually executed synchronously, like
 * software interrupts.
 *
 * Signals are very low overhead.  Using them exclusively to wait
 * for multiple asynchronous events results in very simple dispatch
 * logic with low processor and resource usage.
 *
 * The "event" module is a higher-level interface that can optionally
 * deliver signals to processes.  Messages provide even higher-level
 * IPC services built on signals.  Semaphore arbitration is also
 * implemented using signals.
 *
 * In this implementation, each process has a limited set of signal
 * bits (usually 32) and can wait for multiple signals at the same
 * time using sig_wait().  Signals can also be polled using sig_check(),
 * but a process spinning on its signals usually defeats their purpose
 * of providing a multitasking-friendly infrastructure for event-driven
 * applications.
 *
 * Signals are like flags: they are either active or inactive.  After an
 * external event has delivered a particular signal, it remains raised until
 * the process acknowledges it using either sig_wait() or sig_check().
 * Counting signals is not a reliable way to count how many times a
 * particular event has occurred, because the same signal may be
 * delivered twice before the process can notice.
 *
 * Signals can be delivered synchronously via sig_send() or asynchronously via
 * sig_post().
 *
 * In the synchronous case the process is awakened if it was waiting for any
 * signal and immediately dispatched for execution via a direct context switch,
 * if its priority is greater than the running process.
 *
 * <pre>
 * - Synchronous-signal delivery:
 *
 *     P1__                                   __P2
 *         \                                 /
 *          \__sig_send()____proc_wakeup()__/
 * </pre>
 *
 * In the asynchronous case, the process is scheduled for execution as a
 * consequence of the delivery, but it will be dispatched by the scheduler as
 * usual, according to the scheduling policy.
 *
 * <pre>
 * - Asynchronous-signal delivery:
 *
 *     P1__                  __P1__                       __P2
 *         \                /      \                     /
 *          \__sig_post()__/        \__proc_schedule()__/
 * </pre>
 *
 * In this way, any execution context, including an interrupt handler, can
 * deliver a signal to a process. However, synchronous signal delivery from a
 * non-sleepable context (like an interrupt handler) is forbidden in order to
 * avoid potential deadlock conditions. Instead, sig_post() can be used from
 * any context, expecially from interrupt context or when the preemption is
 * disabled.
 *
 * Multiple independent signals may be delivered at once with a single
 * invocation of sig_send() or sig_post(), although this is rarely useful.
 *
 * \section signal_allocation Signal Allocation
 *
 * There's no hardcoded mapping of specific events to signal bits.
 * The meaning of a particular signal bit is defined by an agreement
 * between the delivering entity and the receiving process.
 * For instance, a terminal driver may be designed to deliver
 * a signal bit called SIG_INT when it reads the CTRL-C sequence
 * from the keyboard, and a process may react to it by quitting.
 *
 * \section sig_single SIG_SINGLE
 *
 * The SIG_SINGLE bit is reserved as a convenient shortcut in those
 * simple scenarios where a process needs to wait on just one event
 * synchronously.  By using SIG_SINGLE, there's no need to allocate
 * a specific signal from the free pool.  The constraints for safely
 * accessing SIG_SINGLE are:
 *  - The process MUST sig_wait() exclusively on SIG_SINGLE
 *  - SIG_SIGNAL MUST NOT be left pending after use (sig_wait() will reset
 *        it automatically)
 *  - Do not sleep between starting the asynchronous task that will fire
 *    SIG_SINGLE, and the call to  sig_wait().
 *  - Do not call system functions that may implicitly sleep, such as
 *    timer_delayTicks().
 *
 * \version $Id$
 * \author Bernie Innocenti <bernie@codewiz.org>
 */

#include "signal.h"

#include "cfg/cfg_timer.h"
#include <cfg/debug.h>
#include <cfg/depend.h>

#include <cpu/irq.h>
#include <kern/proc.h>
#include <kern/proc_p.h>


#if CONFIG_KERN_SIGNALS

// Check config dependencies
CONFIG_DEPEND(CONFIG_KERN_SIGNALS, CONFIG_KERN);

/**
 * Check if any of the signals in \a sigs has occurred and clear them.
 *
 * \return the signals that have occurred.
 */
sigmask_t sig_check(sigmask_t sigs)
{
        sigmask_t result;
        cpu_flags_t flags;

        IRQ_SAVE_DISABLE(flags);
        result = current_process->sig_recv & sigs;
        current_process->sig_recv &= ~sigs;
        IRQ_RESTORE(flags);

        return result;
}


/**
 * Sleep until any of the signals in \a sigs occurs.
 * \return the signal(s) that have awoken the process.
 */
sigmask_t sig_wait(sigmask_t sigs)
{
        sigmask_t result;

        /* Sleeping with IRQs disabled or preemption forbidden is illegal */
        IRQ_ASSERT_ENABLED();
        ASSERT(proc_preemptAllowed());

        /*
         * This is subtle: there's a race condition where a concurrent process
         * or an interrupt may call sig_send()/sig_post() to set a bit in
         * Process.sig_recv just after we have checked for it, but before we've
         * set Process.sig_wait to let them know we want to be awaken.
         *
         * In this case, we'd deadlock with the signal bit already set and the
         * process never being reinserted into the ready list.
         */
        IRQ_DISABLE;

        /* Loop until we get at least one of the signals */
        while (!(result = current_process->sig_recv & sigs))
        {
                /*
                 * Tell "them" that we want to be awaken when any of these
                 * signals arrives.
                 */
                current_process->sig_wait = sigs;

                /* Go to sleep and proc_switch() to another process. */
                proc_switch();
                /*
                 * When we come back here, the wait mask must have been
                 * cleared by someone through sig_send()/sig_post(), and at
                 * least one of the signals we were expecting must have been
                 * delivered to us.
                 */
                ASSERT(!current_process->sig_wait);
                ASSERT(current_process->sig_recv & sigs);
        }

        /* Signals found: clear them and return */
        current_process->sig_recv &= ~sigs;

        IRQ_ENABLE;
        return result;
}

#if CONFIG_TIMER_EVENTS

#include <drv/timer.h>
/**
 * Sleep until any of the signals in \a sigs or \a timeout ticks elapse.
 * If the timeout elapse a SIG_TIMEOUT is added to the received signal(s).
 * \return the signal(s) that have awoken the process.
 * \note Caller must check return value to check which signal awoke the process.
 */
sigmask_t sig_waitTimeout(sigmask_t sigs, ticks_t timeout)
{
        Timer t;
        sigmask_t res;
        cpu_flags_t flags;

        ASSERT(!sig_check(SIG_TIMEOUT));
        ASSERT(!(sigs & SIG_TIMEOUT));
        /* IRQ are needed to run timer */
        ASSERT(IRQ_ENABLED());

        timer_set_event_signal(&t, proc_current(), SIG_TIMEOUT);
        timer_setDelay(&t, timeout);
        timer_add(&t);
        res = sig_wait(SIG_TIMEOUT | sigs);

        IRQ_SAVE_DISABLE(flags);
        /* Remove timer if sigs occur before timer signal */
        if (!(res & SIG_TIMEOUT) && !sig_check(SIG_TIMEOUT))
                timer_abort(&t);
        IRQ_RESTORE(flags);
        return res;
}

#endif // CONFIG_TIMER_EVENTS

INLINE void __sig_signal(Process *proc, sigmask_t sigs, bool wakeup)
{
        cpu_flags_t flags;

        if (UNLIKELY(proc == current_process))
                return;

        IRQ_SAVE_DISABLE(flags);

        /* Set the signals */
        proc->sig_recv |= sigs;

        /* Check if process needs to be awoken */
        if (proc->sig_recv & proc->sig_wait)
        {
                proc->sig_wait = 0;
                if (wakeup)
                        proc_wakeup(proc);
                else
                        SCHED_ENQUEUE_HEAD(proc);
        }
        IRQ_RESTORE(flags);
}

/**
 * Send the signals \a sigs to the process \a proc and immeditaly dispatch it
 * for execution.
 *
 * The process will be awoken if it was waiting for any of them and immediately
 * dispatched for execution.
 *
 * \note This function can't be called from IRQ context, use sig_post()
 * instead.
 */
void sig_send(Process *proc, sigmask_t sigs)
{
        ASSERT_USER_CONTEXT();
        IRQ_ASSERT_ENABLED();
        ASSERT(proc_preemptAllowed());

        __sig_signal(proc, sigs, true);
}

/**
 * Send the signals \a sigs to the process \a proc.
 * The process will be awoken if it was waiting for any of them.
 *
 * \note This call is interrupt safe.
 */
void sig_post(Process *proc, sigmask_t sigs)
{
        __sig_signal(proc, sigs, false);
}

#endif /* CONFIG_KERN_SIGNALS */