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    <title>Timeouts</title>
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    <title>Timeouts</title>
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    <para>Kernel subsystems can register a callback function to be executed
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    <para>Kernel subsystems can register a callback function to be executed
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    with a specified delay. Such a registration is represented by a kernel
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    with a specified delay. Such a registration is represented by a kernel
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    structure called <classname>timeout</classname>. Timeouts are registered
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    structure called <classname>timeout</classname>. Timeouts are registered
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    via <code>timeout_register</code> function. This function takes a pointer
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    via <code>timeout_register()</code> function. This function takes a pointer
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    to a timeout structure, a callback function, a parameter of the callback
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    to a timeout structure, a callback function, a parameter of the callback
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    function and a delay in microseconds as parameters. After the structure is
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    function and a delay in microseconds as parameters. After the structure is
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    initialized with all these values, it is sorted into the processor's list
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    initialized with all these values, it is sorted into the processor's list
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    of active timeouts, according to the number of clock interrupts remaining
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    of active timeouts, according to the number of clock interrupts remaining
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    to their expiration and relatively to already listed timeouts.</para>
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    to their expiration and relatively to already listed timeouts.</para>
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    <para>Timeouts can be unregistered via <code>timeout_unregister</code>.
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    <para>Timeouts can be unregistered via <code>timeout_unregister()</code>.
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    This function can, as opposed to <code>timeout_register</code>, fail when
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    This function can, as opposed to <code>timeout_register()</code>, fail when
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    it is too late to remove the timeout from the list of active
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    it is too late to remove the timeout from the list of active
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    timeouts.</para>
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    timeouts.</para>
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    <para>Timeouts are nearing their expiration in the list of active timeouts
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    <para>Timeouts are nearing their expiration in the list of active timeouts
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    which exists on every processor in the system. The expiration counters are
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    which exists on every processor in the system. The expiration counters are
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  <section>
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  <section>
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    <title>Generic clock interrupt handler</title>
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    <title>Generic clock interrupt handler</title>
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    <para>On each clock interrupt, the architecture specific part of the clock
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    <para>On each clock interrupt, the architecture specific part of the clock
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    interrupt handler makes a call to the generic clock interrupt handler
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    interrupt handler makes a call to the generic clock interrupt handler
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    implemented by the <code>clock</code> function. The generic handler takes
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    implemented by the <code>clock()</code> function. The generic handler takes
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    care of several mission critical goals:</para>
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    care of several mission critical goals:</para>
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    <itemizedlist>
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    <itemizedlist>
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      <listitem>
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      <listitem>
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        <para>expiration of timeouts,</para>
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        <para>expiration of timeouts,</para>
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      <listitem>
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      <listitem>
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        <para>preemption of threads.</para>
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        <para>preemption of threads.</para>
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      </listitem>
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      </listitem>
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    </itemizedlist>
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    </itemizedlist>
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    <para>The <code>clock</code> function checks for expired timeouts and
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    <para>The <code>clock()</code> function checks for expired timeouts and
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    decrements unexpired timeout expiration counters exactly one more times
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    decrements unexpired timeout expiration counters exactly one more times
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    than is the number of missed clock signals (i.e. at least once and
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    than is the number of missed clock signals (i.e. at least once and
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    possibly more times, depending on the missed clock signals counter). The
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    possibly more times, depending on the missed clock signals counter). The
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    time of the day counters are also updated one more times than is the
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    time of the day counters are also updated one more times than is the
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    number of missed clock signals. And finally, the remaining timeslice of
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    number of missed clock signals. And finally, the remaining timeslice of