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| author | Austin Clements <[email protected]> | 2011-09-07 11:49:14 -0400 |
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| committer | Austin Clements <[email protected]> | 2011-09-07 11:49:14 -0400 |
| commit | 01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1 (patch) | |
| tree | 4320eb3d09f31f4a628b80d45482a72ee7c3956b /web/xv6-sleep.html | |
| parent | 64a03bd7aa5c03a626a2da4730a45fcceea75322 (diff) | |
| download | xv6-labs-01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1.tar.gz xv6-labs-01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1.tar.bz2 xv6-labs-01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1.zip | |
Remove web directory; all cruft or moved to 6.828 repo
Diffstat (limited to 'web/xv6-sleep.html')
| -rw-r--r-- | web/xv6-sleep.html | 100 |
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diff --git a/web/xv6-sleep.html b/web/xv6-sleep.html deleted file mode 100644 index e712a40..0000000 --- a/web/xv6-sleep.html +++ /dev/null @@ -1,100 +0,0 @@ -<title>Homework: sleep and wakeup</title> -<html> -<head> -</head> -<body> - -<h1>Homework: sleep and wakeup</h1> - -<p> -<b>Read</b>: pipe.c - -<p> -<b>Hand-In Procedure</b> -<p> -You are to turn in this homework at the beginning of lecture. Please -write up your answers to the questions below and hand them in to a -6.828 staff member at the beginning of lecture. -<p> -<b>Introduction</b> -<p> - -Remember in lecture 7 we discussed locking a linked list implementation. -The insert code was: - -<pre> - struct list *l; - l = list_alloc(); - l->next = list_head; - list_head = l; -</pre> - -and if we run the insert on multiple processors simultaneously with no locking, -this ordering of instructions can cause one of the inserts to be lost: - -<pre> - CPU1 CPU2 - - struct list *l; - l = list_alloc(); - l->next = list_head; - struct list *l; - l = list_alloc(); - l->next = list_head; - list_head = l; - list_head = l; -</pre> - -(Even though the instructions can happen simultaneously, we -write out orderings where only one CPU is "executing" at a time, -to avoid complicating things more than necessary.) -<p> - -In this case, the list element allocated by CPU2 is lost from -the list by CPU1's update of list_head. -Adding a lock that protects the final two instructions makes -the read and write of list_head atomic, so that this -ordering is impossible. -<p> - -The reading for this lecture is the implementation of sleep and wakeup, -which are used for coordination between different processes executing -in the kernel, perhaps simultaneously. -<p> - -If there were no locking at all in sleep and wakeup, it would be -possible for a sleep and its corresponding wakeup, if executing -simultaneously on different processors, to miss each other, -so that the wakeup didn't find any process to wake up, and yet the -process calling sleep does go to sleep, never to awake. Obviously this is something -we'd like to avoid. -<p> - -Read the code with this in mind. - -<p> -<br><br> -<b>Questions</b> -<p> -(Answer and hand in.) -<p> - -1. How does the proc_table_lock help avoid this problem? Give an -ordering of instructions (like the above example for linked list -insertion) -that could result in a wakeup being missed if the proc_table_lock were not used. -You need only include the relevant lines of code. -<p> - -2. sleep is also protected by a second lock, its second argument, -which need not be the proc_table_lock. Look at the example in ide.c, -which uses the ide_lock. Give an ordering of instructions that could -result in a wakeup being missed if the ide_lock were not being used. -(Hint: this should not be the same as your answer to question 2. The -two locks serve different purposes.)<p> - -<br><br> -<b>This completes the homework.</b> - -</body> - |