85 lines
3.6 KiB
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85 lines
3.6 KiB
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<title>Programming with signal handling in mind - SBCL Internals</title>
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<a name="Programming-with-signal-handling-in-mind"></a>
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Previous: <a rel="previous" accesskey="p" href="Implementation-warts.html#Implementation-warts">Implementation warts</a>,
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Up: <a rel="up" accesskey="u" href="Signal-handling.html#Signal-handling">Signal handling</a>
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<h3 class="section">6.4 Programming with signal handling in mind</h3>
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<h4 class="subsection">6.4.1 On reentrancy</h4>
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<p>Since they might be invoked in the middle of just about anything,
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signal handlers must invoke only reentrant functions or async signal
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safe functions to be more precise. Functions passed to
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<code>INTERRUPT-THREAD</code> have the same restrictions and considerations
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as signal handlers.
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<p>Destructive modification, and holding mutexes to protect desctructive
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modifications from interfering with each other are often the cause of
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non-reentrancy. Recursive locks are not likely to help, and while
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<code>WITHOUT-INTERRUPTS</code> is, it is considered untrendy to litter the
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code with it.
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<p>Some basic functionality, such as streams and the debugger are
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intended to be reentrant, but not much effort has been spent on
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verifying it.
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<h4 class="subsection">6.4.2 More deadlocks</h4>
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<p>If functions A and B directly or indirectly lock mutexes M and N, they
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should do so in the same order to avoid deadlocks.
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<p>A less trivial scenario is where there is only one lock involved but
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it is acquired in a <code>WITHOUT-GCING</code> in thread A, and outside of
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<code>WITHOUT-GCING</code> in thread B. If thread A has entered
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<code>WITHOUT-GCING</code> but thread B has the lock when the gc hits, then
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A cannot leave <code>WITHOUT-GCING</code> because it is waiting for the lock
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the already suspended thread B has. From this scenario one can easily
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derive the rule: in a <code>WITHOUT-GCING</code> form (or pseudo atomic for
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that matter) never wait for another thread that's not in
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<code>WITHOUT-GCING</code>.
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<h4 class="subsection">6.4.3 Calling user code</h4>
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<p>For the reasons above, calling user code, i.e. functions passed in, or
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in other words code that one cannot reason about, from non-reentrant
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code (holding locks), <code>WITHOUT-INTERRUPTS</code>, <code>WITHOUT-GCING</code>
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is dangerous and best avoided.
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