83 lines
3.5 KiB
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83 lines
3.5 KiB
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<title>Binding and unbinding - SBCL Internals</title>
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This manual is part of the SBCL software system. See the `README'
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<div class="node">
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<p>
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<a name="Binding-and-unbinding"></a>
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Previous: <a rel="previous" accesskey="p" href="Overview.html#Overview">Overview</a>,
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Up: <a rel="up" accesskey="u" href="Specials.html#Specials">Specials</a>
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<hr>
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</div>
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<h3 class="section">8.2 Binding and unbinding</h3>
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<p>Binding goes like this: the binding stack pointer (bsp) is bumped, old
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value and symbol are stored at bsp - 1, new value is stored in
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symbol's value slot or the tls.
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<p>Unbinding: the symbol's value is restored from bsp - 1, value and
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symbol at bsp - 1 are set to zero, and finally bsp is decremented.
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<p>The <code>UNBIND-TO-HERE</code> VOP assists in unwinding the stack. It
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iterates over the bindings on the binding stack until it reaches the
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prescribed point. For each binding with a non-zero symbol it does an
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<code>UNBIND</code>.
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<p>How can a binding's symbol be zero? <code>BIND</code> is not pseudo atomic
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(for performance reasons) and it can be interrupted by a signal. If
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the signal hits after the bsp is incremented but before the values on
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the stack are set the symbol is zero because a thread starts with a
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zeroed tls plus <code>UNBIND</code> and <code>UNBIND-TO-HERE</code> both zero the
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binding being unbound.
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<p>Zeroing the binding's symbol would not be enough as the binding's
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value can be moved or garbage collected and if the above interrupt
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initiates gc (or be <code>SIG_STOP_FOR_GC</code>) it will be greeted by a
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garbage pointer.
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<p>Furthermore, <code>BIND</code> must always write the value to the binding
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stack first and the symbol second because the symbol being non-zero
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means validity to <code>UNBIND-TO-HERE</code>. For similar reasons
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<code>UNBIND</code> also zeroes the symbol first. But if it is interrupted
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by a signal that does an async unwind then <code>UNBIND-TO-HERE</code> can
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be triggered when the symbol is zeroed but the value is not. In this
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case <code>UNBIND-TO-HERE</code> must zero out the value to avoid leaving
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garbage around that may wreck the ship on the next <code>BIND</code>.
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<p>In other words, the invariant is that the binding stack above bsp only
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contains zeros. This makes <code>BIND</code> safe in face of gc triggered at
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any point during its execution.
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</body></html>
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