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  <section>

  <section>

    <title>Active kernel primitives</title>

    <title>Active kernel primitives</title>

    <section>

    <section>

      <title>Spinlocks</title>

      <title>Spinlocks</title>

      <para>The basic mutual exclusion primitive is the spinlock. The spinlock

      <para>The basic mutual exclusion primitive is the spinlock. The spinlock

      implements active waiting for the availability of a memory lock (i.e.

      implements active waiting for the availability of a memory lock (i.e.

      simple variable) in a multiprocessor-safe manner. This safety is

      simple variable) in a multiprocessor-safe manner. This safety is

  <section>

  <section>

    <title>Passive kernel synchronization</title>

    <title>Passive kernel synchronization</title>

    <section>

    <section>

      <title>Wait queues</title>

      <title>Wait queues</title>

      <para>A wait queue is the basic passive synchronization primitive on

      <para>A wait queue is the basic passive synchronization primitive on

      which all other passive synchronization primitives are built. Simply

      which all other passive synchronization primitives are built. Simply

      put, it allows a thread to sleep until an event associated with the

      put, it allows a thread to sleep until an event associated with the

      characteristic for all passive HelenOS synchronization primitives, with

      characteristic for all passive HelenOS synchronization primitives, with

      the exception as described below.</para>

      the exception as described below.</para>

    </section>

    </section>

    <section>

    <section>

      <title>Semaphores</title>

      <title>Semaphores</title>

      <para>The interesting point about wait queues is that the number of

      <para>The interesting point about wait queues is that the number of

      missed wakeups is equal to the number of threads that will not block in

      missed wakeups is equal to the number of threads that will not block in

      <code>watiq_sleep_timeout</code> and would immediately succeed instead.

      <code>watiq_sleep_timeout</code> and would immediately succeed instead.

    </section>

    </section>

    <section>

    <section>

      <title>Mutexes</title>

      <title>Mutexes</title>

      <para>Mutexes are sometimes referred to as binary sempahores. That means

      <para>Mutexes are sometimes referred to as binary sempahores. That means

      that mutexes are like semaphores that allow only one thread in its

      that mutexes are like semaphores that allow only one thread in its

      critical section. Indeed, mutexes in HelenOS are implemented exactly in

      critical section. Indeed, mutexes in HelenOS are implemented exactly in

      this way: they are built on top of semaphores. From another point of

      this way: they are built on top of semaphores. From another point of

      view, they can be viewed as spinlocks without busy waiting. Their

      view, they can be viewed as spinlocks without busy waiting. Their

    </section>

    </section>

    <section>

    <section>

      <title>Reader/writer locks</title>

      <title>Reader/writer locks</title>

      <para>Reader/writer locks, or rwlocks, are by far the most complicated

      <para>Reader/writer locks, or rwlocks, are by far the most complicated

      synchronization primitive within the kernel. The goal of these locks is

      synchronization primitive within the kernel. The goal of these locks is

      to improve concurrency of applications, in which threads need to

      to improve concurrency of applications, in which threads need to

      synchronize access to a shared resource, and that access can be

      synchronize access to a shared resource, and that access can be

      partitioned into a read-only mode and a write mode. Reader/writer locks

      partitioned into a read-only mode and a write mode. Reader/writer locks

    </section>

    </section>

    <section>

    <section>

      <title>Condition variables</title>

      <title>Condition variables</title>

      <para>Condition variables can be used for waiting until a condition

      <para>Condition variables can be used for waiting until a condition

      becomes true. In this respect, they are similar to wait queues. But

      becomes true. In this respect, they are similar to wait queues. But

      contrary to wait queues, condition variables have different semantics

      contrary to wait queues, condition variables have different semantics

      that allows events to be lost when there is no thread waiting for them.

      that allows events to be lost when there is no thread waiting for them.

      In order to support this, condition variables don't use direct hand-off

      In order to support this, condition variables don't use direct hand-off

      and operate in a way similar to the example below. A thread waiting for

      and operate in a way similar to the example below. A thread waiting for

      the condition becoming true does the following:</para>

      the condition becoming true does the following:</para>

      <example>

      <example>

while (!<varname>condition</varname>)

while (!<varname>condition</varname>)

        <function>condvar_wait_timeout</function>(<varname>cv</varname>, <varname>mtx</varname>);

        <function>condvar_wait_timeout</function>(<varname>cv</varname>, <varname>mtx</varname>);

/* <remark>the condition is true, do something</remark> */

/* <remark>the condition is true, do something</remark> */

<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting>

<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting>

      <para>A thread that causes the condition become true signals this event

      <para>A thread that causes the condition become true signals this event

      like this:</para>

      like this:</para>

      <example>

      <example>

<varname>condition</varname> = <constant>true</constant>;

<varname>condition</varname> = <constant>true</constant>;

<function>condvar_signal</function>(<varname>cv</varname>);  /* <remark>condvar_broadcast(cv);</remark> */

<function>condvar_signal</function>(<varname>cv</varname>);  /* <remark>condvar_broadcast(cv);</remark> */

      <para>The wait operation, <code>condvar_wait_timeout</code>, always puts

      <para>The wait operation, <code>condvar_wait_timeout</code>, always puts

      the calling thread to sleep. The thread then sleeps until another thread

      the calling thread to sleep. The thread then sleeps until another thread

      invokes <code>condvar_broadcast</code> on the same condition variable or

      invokes <code>condvar_broadcast</code> on the same condition variable or

      until it is woken up by <code>condvar_signal</code>. The

      until it is woken up by <code>condvar_signal</code>. The

    <title>Userspace synchronization</title>

    <title>Userspace synchronization</title>

    <section>

    <section>

      <title>Futexes</title>

      <title>Futexes</title>

      <para></para>

      <para></para>

    </section>

    </section>

  </section>

  </section>

</chapter>

</chapter>