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<?xml version="1.0" encoding="UTF-8"?>
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<?xml version="1.0" encoding="UTF-8"?>
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<chapter id="sync">
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<chapter id="sync">
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  <?dbhtml filename="sync.html"?>
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  <?dbhtml filename="sync.html"?>
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  <title>Mutual exclusion and synchronization</title>
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  <title>Synchronization</title>
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  <section>
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  <section>
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    <title>Introduction</title>
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    <title>Introduction</title>
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    <para>The HelenOS operating system is designed to make use of the
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    <para>The HelenOS operating system is designed to make use of the
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      win the futex to sleep until the winner wakes one of them up.</para>
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      win the futex to sleep until the winner wakes one of them up.</para>
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      <para>A futex should be initialized by setting its userspace counter to
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      <para>A futex should be initialized by setting its userspace counter to
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      one before it is used. When locking the futex via userspace library
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      one before it is used. When locking the futex via userspace library
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      function <code>futex_down_timeout</code>, the library code atomically
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      function <code>futex_down_timeout</code>, the library code atomically
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      decrements the futex counter and tests if it droped below zero. If it
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      decrements the futex counter and tests if it dropped below zero. If it
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      did, then the futex is locked by another thread and the library uses the
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      did, then the futex is locked by another thread and the library uses the
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      <constant>SYS_FUTEX_SLEEP</constant> syscall to put the thread asleep.
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      <constant>SYS_FUTEX_SLEEP</constant> syscall to put the thread asleep.
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      If the counter decreased to 0, then there was no contention and the
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      If the counter decreased to 0, then there was no contention and the
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      thread can enter the critical section protected by the futex. When the
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      thread can enter the critical section protected by the futex. When the
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      thread later leaves that critical section, it, using library function
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      thread later leaves that critical section, it, using library function