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82 lines
3.5 KiB
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<title>The deferral mechanism - SBCL Internals</title>
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<p>
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<a name="The-deferral-mechanism"></a>
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Next: <a rel="next" accesskey="n" href="Implementation-warts.html#Implementation-warts">Implementation warts</a>,
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Previous: <a rel="previous" accesskey="p" href="Groups-of-signals.html#Groups-of-signals">Groups of signals</a>,
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Up: <a rel="up" accesskey="u" href="Signal-handling.html#Signal-handling">Signal handling</a>
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</div>
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<h3 class="section">6.2 The deferral mechanism</h3>
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<h4 class="subsection">6.2.1 Pseudo atomic sections</h4>
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<p>Some operations, such as allocation, consist of several steps and
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temporarily break for instance gc invariants. Interrupting said
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operations is therefore dangerous to one's health. Blocking the
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signals for each allocation is out of question as the overhead of the
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two <code>sigsetmask</code> system calls would be enormous. Instead, pseudo
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atomic sections are implemented with a simple flag.
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<p>When a deferrable signal is delivered to a thread within a pseudo
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atomic section the pseudo-atomic-interrupted flag is set, the signal
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and its context are stored, and all deferrable signals blocked. This
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is to guarantee that there is at most one pending handler in
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SBCL. While the signals are blocked, the responsibilty of keeping
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track of other pending signals lies with the OS.
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<p>On leaving the pseudo atomic section, the pending handler is run and
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the signals are unblocked.
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<h4 class="subsection">6.2.2 <code>WITHOUT-INTERRUPTS</code></h4>
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<p>Similar to pseudo atomic, <code>WITHOUT-INTERRUPTS</code> defers deferrable
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signals in its thread until the end of its body, provided it is not
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nested in another <code>WITHOUT-INTERRUPTS</code>.
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<p>Not so frequently used as pseudo atomic, <code>WITHOUT-INTERRUPTS</code>
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benefits less from the deferral mechanism.
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<h4 class="subsection">6.2.3 Stop the world</h4>
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<p>Something of a special case, a signal that is blockable but not
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deferrable by <code>WITHOUT-INTERRUPTS</code> is <code>SIG_STOP_FOR_GC</code>. It
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is deferred by pseudo atomic and <code>WITHOUT-GCING</code>.
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