nuttx/sched/sched/sched_lock.c

/****************************************************************************
 * sched/sched/sched_lock.c
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.  The
 * ASF licenses this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the
 * License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#include <sys/types.h>
#include <sched.h>
#include <assert.h>

#include <arch/irq.h>

#include <nuttx/irq.h>
#include <nuttx/arch.h>
#include <nuttx/sched_note.h>

#include "sched/sched.h"

/****************************************************************************
 * Public Data
 ****************************************************************************/

/* Pre-emption is disabled via the interface sched_lock(). sched_lock()
 * works by preventing context switches from the currently executing tasks.
 * This prevents other tasks from running (without disabling interrupts) and
 * gives the currently executing task exclusive access to the (single) CPU
 * resources. Thus, sched_lock() and its companion, sched_unlock(), are
 * used to implement some critical sections.
 *
 * In the single CPU case, pre-emption is disabled using a simple lockcount
 * in the TCB. When the scheduling is locked, the lockcount is incremented;
 * when the scheduler is unlocked, the lockcount is decremented. If the
 * lockcount for the task at the head of the g_readytorun list has a
 * lockcount > 0, then pre-emption is disabled.
 *
 * No special protection is required since only the executing task can
 * modify its lockcount.
 */

#ifdef CONFIG_SMP
/* Used to keep track of which CPU(s) hold the IRQ lock. */

volatile cpu_set_t g_cpu_lockset;

#endif /* CONFIG_SMP */

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name:  sched_lock
 *
 * Description:
 *   This function disables context switching by disabling addition of
 *   new tasks to the g_readytorun task list.  The task that calls this
 *   function will be the only task that is allowed to run until it
 *   either calls  sched_unlock() (the appropriate number of times) or
 *   until it blocks itself.
 *
 * Input Parameters:
 *   None
 *
 * Returned Value:
 *   OK on success; ERROR on failure
 *
 ****************************************************************************/

#ifdef CONFIG_SMP

int sched_lock(void)
{
  FAR struct tcb_s *rtcb;
  int cpu;

  /* If the CPU supports suppression of interprocessor interrupts, then
   * simple disabling interrupts will provide sufficient protection for
   * the following operation.
   */

  rtcb = this_task();

  /* Check for some special cases:  (1) rtcb may be NULL only during early
   * boot-up phases, and (2) sched_lock() should have no effect if called
   * from the interrupt level.
   */

  if (rtcb != NULL && !up_interrupt_context())
    {
      irqstate_t flags;

      /* Catch attempts to increment the lockcount beyond the range of the
       * integer type.
       */

      DEBUGASSERT(rtcb->lockcount < MAX_LOCK_COUNT);

      flags = enter_critical_section();
      cpu = this_cpu();

      /* We must hold the lock on this CPU before we increment the lockcount
       * for the first time. Holding the lock is sufficient to lockout
       * context switching.
       */

      if (rtcb->lockcount == 0)
        {
          /* We don't have the scheduler locked.  But logic running on a
           * different CPU may have the scheduler locked.  It is not
           * possible for some other task on this CPU to have the scheduler
           * locked (or we would not be executing!).
           */

          DEBUGASSERT((g_cpu_lockset & (1 << cpu)) == 0);
          g_cpu_lockset |= (1 << cpu);
        }
      else
        {
          /* If this thread already has the scheduler locked, then
           * g_cpu_lockset should indicate that the scheduler is locked
           * and g_cpu_lockset should include the bit setting for this CPU.
           */

          DEBUGASSERT((g_cpu_lockset & (1 << cpu)) != 0);
        }

      /* A counter is used to support locking.  This allows nested lock
       * operations on this thread (on any CPU)
       */

      rtcb->lockcount++;

#if defined(CONFIG_SCHED_INSTRUMENTATION_PREEMPTION) || \
    defined(CONFIG_SCHED_CRITMONITOR)
      /* Check if we just acquired the lock */

      if (rtcb->lockcount == 1)
        {
          /* Note that we have pre-emption locked */

#ifdef CONFIG_SCHED_CRITMONITOR
          nxsched_critmon_preemption(rtcb, true);
#endif
#ifdef CONFIG_SCHED_INSTRUMENTATION_PREEMPTION
          sched_note_premption(rtcb, true);
#endif
        }
#endif

      /* Move any tasks in the ready-to-run list to the pending task list
       * where they will not be available to run until the scheduler is
       * unlocked and nxsched_merge_pending() is called.
       */

      nxsched_merge_prioritized(list_readytorun(),
                                list_pendingtasks(),
                                TSTATE_TASK_PENDING);

      leave_critical_section(flags);
    }

  return OK;
}

#else /* CONFIG_SMP */

int sched_lock(void)
{
  FAR struct tcb_s *rtcb = this_task();

  /* Check for some special cases:  (1) rtcb may be NULL only during early
   * boot-up phases, and (2) sched_lock() should have no effect if called
   * from the interrupt level.
   */

  if (rtcb != NULL && !up_interrupt_context())
    {
      /* Catch attempts to increment the lockcount beyond the range of the
       * integer type.
       */

      DEBUGASSERT(rtcb->lockcount < MAX_LOCK_COUNT);

      /* A counter is used to support locking.  This allows nested lock
       * operations on this thread (on any CPU)
       */

      rtcb->lockcount++;

#if defined(CONFIG_SCHED_INSTRUMENTATION_PREEMPTION) || \
    defined(CONFIG_SCHED_CRITMONITOR)
      /* Check if we just acquired the lock */

      if (rtcb->lockcount == 1)
        {
          /* Note that we have pre-emption locked */

#ifdef CONFIG_SCHED_CRITMONITOR
          nxsched_critmon_preemption(rtcb, true);
#endif
#ifdef CONFIG_SCHED_INSTRUMENTATION_PREEMPTION
          sched_note_premption(rtcb, true);
#endif
        }
#endif
    }

  return OK;
}

#endif /* CONFIG_SMP */