101 lines
3.1 KiB
HTML
101 lines
3.1 KiB
HTML
<title>Homework: Locking</title>
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<html>
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<head>
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</head>
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<body>
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<h1>Homework: Locking</h1>
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<p>
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<b>Read</b>: spinlock.c
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<p>
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<b>Hand-In Procedure</b>
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<p>
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You are to turn in this homework at the beginning of lecture. Please
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write up your answers to the exercises below and hand them in to a
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6.828 staff member at the beginning of lecture.
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<p>
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<b>Assignment</b>:
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In this assignment we will explore some of the interaction
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between interrupts and locking.
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<p>
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Make sure you understand what would happen if the kernel executed
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the following code snippet:
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<pre>
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struct spinlock lk;
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initlock(&lk, "test lock");
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acquire(&lk);
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acquire(&lk);
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</pre>
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(Feel free to use Bochs to find out. <code>acquire</code> is in <code>spinlock.c</code>.)
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<p>
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An <code>acquire</code> ensures interrupts are off
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on the local processor using <code>cli</code>,
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and interrupts remain off until the <code>release</code>
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of the last lock held by that processor
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(at which point they are enabled using <code>sti</code>).
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<p>
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Let's see what happens if we turn on interrupts while
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holding the <code>ide</code> lock.
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In <code>ide_rw</code> in <code>ide.c</code>, add a call
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to <code>sti()</code> after the <code>acquire()</code>.
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Rebuild the kernel and boot it in Bochs.
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Chances are the kernel will panic soon after boot; try booting Bochs a few times
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if it doesn't.
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<p>
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<b>Turn in</b>: explain in a few sentences why the kernel panicked.
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You may find it useful to look up the stack trace
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(the sequence of <code>%eip</code> values printed by <code>panic</code>)
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in the <code>kernel.asm</code> listing.
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<p>
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Remove the <code>sti()</code> you added,
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rebuild the kernel, and make sure it works again.
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<p>
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Now let's see what happens if we turn on interrupts
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while holding the <code>kalloc_lock</code>.
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In <code>kalloc()</code> in <code>kalloc.c</code>, add
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a call to <code>sti()</code> after the call to <code>acquire()</code>.
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You will also need to add
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<code>#include "x86.h"</code> at the top of the file after
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the other <code>#include</code> lines.
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Rebuild the kernel and boot it in Bochs.
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It will not panic.
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<p>
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<b>Turn in</b>: explain in a few sentences why the kernel didn't panic.
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What is different about <code>kalloc_lock</code>
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as compared to <code>ide_lock</code>?
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<p>
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You do not need to understand anything about the details of the IDE hardware
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to answer this question, but you may find it helpful to look
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at which functions acquire each lock, and then at when those
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functions get called.
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<p>
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(There is a very small but non-zero chance that the kernel will panic
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with the extra <code>sti()</code> in <code>kalloc</code>.
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If the kernel <i>does</i> panic, make doubly sure that
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you removed the <code>sti()</code> call from
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<code>ide_rw</code>. If it continues to panic and the
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only extra <code>sti()</code> is in <code>bio.c</code>,
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then mail <i>6.828-staff@pdos.csail.mit.edu</i>
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and think about buying a lottery ticket.)
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<p>
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<b>Turn in</b>: Why does <code>release()</code> clear
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<code>lock->pcs[0]</code> and <code>lock->cpu</code>
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<i>before</i> clearing <code>lock->locked</code>?
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Why not wait until after?
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</body>
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</html>
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