@func{main}, which never returns.
There's one more trick: the Pintos kernel command line
-is in stored the boot loader. The @command{pintos} program actually
+is stored in the boot loader. The @command{pintos} program actually
modifies a copy of the boot loader on disk each time it runs the kernel,
putting
in whatever command line arguments the user supplies to the kernel,
the loader stored it and put it into the @code{ram_pages} variable for
later use.
-Next, @func{thread_init} initializes the thread system. We will defer
-full discussion to our discussion of Pintos threads below. It is
-called so early in initialization because the console, initialized
-just afterward, tries to use locks, and locks in turn require there to be a
-running thread.
-
-Then we initialize the console so that @func{printf} will work.
-@func{main} calls @func{vga_init}, which initializes the VGA text
-display and clears the screen. It also calls @func{serial_init_poll}
-to initialize the first serial port in ``polling mode,'' that is,
-where the kernel busy-waits for the port to be ready for each
-character to be output. (We use polling mode until we're ready to enable
-interrupts, later.) Finally we initialize the console device and
-print a startup message to the console.
-
-@func{main} calls @func{read_command_line} to break the kernel command
+Next, @func{main} calls @func{read_command_line} to break the kernel command
line into arguments, then @func{parse_options} to read any options at
the beginning of the command line. (Actions specified on the
command line execute later.)
-@func{main} calls @func{random_init} to initialize the kernel random
-number generator. If the user specified @option{-rs} on the
-@command{pintos} command line, @func{parse_options} already did
-this, but calling it a second time is harmless.
+@func{thread_init} initializes the thread system. We will defer full
+discussion to our discussion of Pintos threads below. It is called so
+early in initialization because a valid thread structure is a
+prerequisite for acquiring a lock, and lock acquisition in turn is
+important to other Pintos subsystems. Then we initialize the console
+and print a startup message to the console.
-The next block of functions we call initialize the kernel's memory
+The next block of functions we call initializes the kernel's memory
system. @func{palloc_init} sets up the kernel page allocator, which
doles out memory one or more pages at a time (@pxref{Page Allocator}).
@func{malloc_init} sets
@func{intr_init} sets up the CPU's @dfn{interrupt descriptor table}
(IDT) to ready it for interrupt handling (@pxref{Interrupt
Infrastructure}), then @func{timer_init} and @func{kbd_init} prepare for
-handling timer interrupts and keyboard interrupts, respectively. In
+handling timer interrupts and keyboard interrupts, respectively.
+@func{input_init} sets up to merge serial and keyboard input into one
+stream. In
projects 2 and later, we also prepare to handle interrupts caused by
user programs using @func{exception_init} and @func{syscall_init}.
* Semaphores::
* Locks::
* Monitors::
-* Memory Barriers::
+* Optimization Barriers::
@end menu
@node Disabling Interrupts
A @dfn{lock} is like a semaphore with an initial value of 1
(@pxref{Semaphores}). A lock's equivalent of ``up'' is called
-``acquire'', and the ``down'' operation is called ``release''.
+``release'', and the ``down'' operation is called ``acquire''.
Compared to a semaphore, a lock has one added restriction: only the
thread that acquires a lock, called the lock's ``owner'', is allowed to
@}
@end example
-@node Memory Barriers
-@subsection Memory Barriers
+Note that @code{BUF_SIZE} must divide evenly into @code{SIZE_MAX + 1}
+for the above code to be completely correct. Otherwise, it will fail
+the first time @code{head} wraps around to 0. In practice,
+@code{BUF_SIZE} would ordinarily be a power of 2.
+
+@node Optimization Barriers
+@subsection Optimization Barriers
@c We should try to come up with a better example.
@c Perhaps something with a linked list?
-Suppose we add a ``feature'' that, whenever a timer interrupt
-occurs, the character in global variable @code{timer_put_char} is
-printed on the console, but only if global Boolean variable
-@code{timer_do_put} is true.
+An @dfn{optimization barrier} is a special statement that prevents the
+compiler from making assumptions about the state of memory across the
+barrier. The compiler will not reorder reads or writes of variables
+across the barrier or assume that a variable's value is unmodified
+across the barrier, except for local variables whose address is never
+taken. In Pintos, @file{threads/synch.h} defines the @code{barrier()}
+macro as an optimization barrier.
-If interrupts are enabled, this code for setting up @samp{x} to be
-printed is clearly incorrect, because the timer interrupt could intervene
-between the two assignments:
+One reason to use an optimization barrier is when data can change
+asynchronously, without the compiler's knowledge, e.g.@: by another
+thread or an interrupt handler. The @func{too_many_loops} function in
+@file{devices/timer.c} is an example. This function starts out by
+busy-waiting in a loop until a timer tick occurs:
@example
-timer_do_put = true; /* INCORRECT CODE */
-timer_put_char = 'x';
+/* Wait for a timer tick. */
+int64_t start = ticks;
+while (ticks == start)
+ barrier ();
@end example
-It might not be as obvious that the following code is just as
-incorrect:
+@noindent
+Without an optimization barrier in the loop, the compiler could
+conclude that the loop would never terminate, because @code{start} and
+@code{ticks} start out equal and the loop itself never changes them.
+It could then ``optimize'' the function into an infinite loop, which
+would definitely be undesirable.
+
+Optimization barriers can be used to avoid other compiler
+optimizations. The @func{busy_wait} function, also in
+@file{devices/timer.c}, is an example. It contains this loop:
@example
-timer_put_char = 'x'; /* INCORRECT CODE */
-timer_do_put = true;
+while (loops-- > 0)
+ barrier ();
@end example
-The reason this second example might be a problem is that the compiler
-is, in general, free to reorder operations when it doesn't have a
-visible reason to keep them in the same order. In this case, the
-compiler doesn't know that the order of assignments is important, so its
-optimization pass is permitted to exchange their order.
-There's no telling whether it will actually do this, and it is possible
-that passing the compiler different optimization flags or changing
-compiler versions will produce different behavior.
-
-The following is @emph{not} a solution, because locks neither prevent
-interrupts nor prevent the compiler from reordering the code within the
-region where the lock is held:
+@noindent
+The goal of this loop is to busy-wait by counting @code{loops} down
+from its original value to 0. Without the barrier, the compiler could
+delete the loop entirely, because it produces no useful output and has
+no side effects. The barrier forces the compiler to pretend that the
+loop body has an important effect.
+
+Finally, optimization barriers can be used to force the ordering of
+memory reads or writes. For example, suppose we add a ``feature''
+that, whenever a timer interrupt occurs, the character in global
+variable @code{timer_put_char} is printed on the console, but only if
+global Boolean variable @code{timer_do_put} is true. The best way to
+set up @samp{x} to be printed is then to use an optimization barrier,
+like this:
@example
-lock_acquire (&timer_lock); /* INCORRECT CODE */
timer_put_char = 'x';
+barrier ();
timer_do_put = true;
-lock_release (&timer_lock);
@end example
-Fortunately, real solutions do exist. One possibility is to
-disable interrupts around the assignments. This does not prevent
-reordering, but it makes the assignments atomic as observed by the
-interrupt handler. It also has the extra runtime cost of disabling and
-re-enabling interrupts:
+Without the barrier, the code is buggy because the compiler is free to
+reorder operations when it doesn't see a reason to keep them in the
+same order. In this case, the compiler doesn't know that the order of
+assignments is important, so its optimizer is permitted to exchange
+their order. There's no telling whether it will actually do this, and
+it is possible that passing the compiler different optimization flags
+or using a different version of the compiler will produce different
+behavior.
+
+Another solution is to disable interrupts around the assignments.
+This does not prevent reordering, but it prevents the interrupt
+handler from intervening between the assignments. It also has the
+extra runtime cost of disabling and re-enabling interrupts:
@example
enum intr_level old_level = intr_disable ();
intr_set_level (old_level);
@end example
-A second possibility is to mark the declarations of
+A second solution is to mark the declarations of
@code{timer_put_char} and @code{timer_do_put} as @samp{volatile}. This
keyword tells the compiler that the variables are externally observable
and restricts its latitude for optimization. However, the semantics of
@samp{volatile} are not well-defined, so it is not a good general
-solution.
+solution. The base Pintos code does not use @samp{volatile} at all.
-Usually, the best solution is to use a compiler feature called a
-@dfn{memory barrier}, a special statement that prevents the compiler
-from reordering memory operations across the barrier. In Pintos,
-@file{threads/synch.h} defines the @code{barrier()} macro as a memory
-barrier. Here's how we would use a memory barrier to fix this code:
+The following is @emph{not} a solution, because locks neither prevent
+interrupts nor prevent the compiler from reordering the code within the
+region where the lock is held:
@example
+lock_acquire (&timer_lock); /* INCORRECT CODE */
timer_put_char = 'x';
-barrier ();
timer_do_put = true;
+lock_release (&timer_lock);
@end example
-The compiler also treats invocation of any function defined externally,
-that is, in another source file, as a limited form of a memory barrier.
-Specifically, the compiler assumes that any externally defined function
-may access any statically or dynamically allocated data and any local
-variable whose address is taken. This often means that explicit
-barriers can be omitted, and, indeed, this is why the base Pintos code
-does not need any barriers.
+The compiler treats invocation of any function defined externally,
+that is, in another source file, as a limited form of optimization
+barrier. Specifically, the compiler assumes that any externally
+defined function may access any statically or dynamically allocated
+data and any local variable whose address is taken. This often means
+that explicit barriers can be omitted. It is one reason that Pintos
+contains few explicit barriers.
A function defined in the same source file, or in a header included by
-the source file, cannot be relied upon as a memory barrier.
+the source file, cannot be relied upon as a optimization barrier.
This applies even to invocation of a function before its
definition, because the compiler may read and parse the entire source
file before performing optimization.
@section Hash Table
Pintos provides a hash table data structure in @file{lib/kernel/hash.c}.
-To use it you will need to manually include its header file,
-@file{lib/kernel/hash.h}, with @code{#include <hash.h>}. Intentionally,
-no code provided with Pintos uses the hash table, which means that you
+To use it you will need to include its header file,
+@file{lib/kernel/hash.h}, with @code{#include <hash.h>}.
+No code provided with Pintos uses the hash table, which means that you
are free to use it as is, modify its implementation for your own
purposes, or ignore it, as you wish.
real element type.
@end deftp
-You will often need to obtain a @struct{hash_elem}
-given a real element of the hash table, and vice versa. Given
-a real element of the hash table, obtaining a pointer to its
-@struct{hash_elem} is trivial: take the address of the
-@struct{hash_elem} member. Use the @code{hash_entry()} macro to go the
-other direction.
+You will often need to obtain a @struct{hash_elem} given a real element
+of the hash table, and vice versa. Given a real element of the hash
+table, you may use the @samp{&} operator to obtain a pointer to its
+@struct{hash_elem}. Use the @code{hash_entry()} macro to go the other
+direction.
@deftypefn {Macro} {@var{type} *} hash_entry (struct hash_elem *@var{elem}, @var{type}, @var{member})
Returns a pointer to the structure that @var{elem}, a pointer to a
For example, suppose @code{h} is a @code{struct hash_elem *} variable
that points to a @struct{thread} member (of type @struct{hash_elem})
-named @code{h_elem}. Then, @code{hash_entry (h, struct thread, h_elem)}
+named @code{h_elem}. Then, @code{hash_entry@tie{}(h, struct thread, h_elem)}
yields the address of the @struct{thread} that @code{h} points within.
@end deftypefn
+@xref{Hash Table Example}, for an example.
+
Each hash table element must contain a key, that is, data that
-identifies and distinguishes elements in the hash table. Every element
-in a hash table at a given time must have a unique key. (Elements may
+identifies and distinguishes elements, which must be unique
+among elements in the hash table. (Elements may
also contain non-key data that need not be unique.) While an element is
-in a hash table, its key data must not be changed. For each hash table,
-you must write two functions that act on keys: a hash function and a
-comparison function. These functions must match the following
-prototypes:
+in a hash table, its key data must not be changed. Instead, if need be,
+remove the element from the hash table, modify its key, then reinsert
+the element.
+
+For each hash table, you must write two functions that act on keys: a
+hash function and a comparison function. These functions must match the
+following prototypes:
@deftp {Type} {unsigned hash_hash_func (const struct hash_elem *@var{element}, void *@var{aux})}
Returns a hash of @var{element}'s data, as a value anywhere in the range
@xref{Hash Auxiliary Data}, for an explanation of @var{aux}.
@end deftp
-A few functions that act on hashes accept a pointer to a third kind of
+@xref{Hash Table Example}, for hash and comparison function examples.
+
+A few functions accept a pointer to a third kind of
function as an argument:
@deftp {Type} {void hash_action_func (struct hash_elem *@var{element}, void *@var{aux})}
@node Basic Hash Functions
@subsection Basic Functions
-These functions create and destroy hash tables and obtain basic
-information about their contents.
+These functions create, destroy, and inspect hash tables.
@deftypefun bool hash_init (struct hash *@var{hash}, hash_hash_func *@var{hash_func}, hash_less_func *@var{less_func}, void *@var{aux})
-Initializes @var{hash} as a hash table using @var{hash_func} as hash
+Initializes @var{hash} as a hash table with @var{hash_func} as hash
function, @var{less_func} as comparison function, and @var{aux} as
auxiliary data.
Returns true if successful, false on failure. @func{hash_init} calls
@deftypefun {struct hash_elem *} hash_insert (struct hash *@var{hash}, struct hash_elem *@var{element})
Searches @var{hash} for an element equal to @var{element}. If none is
found, inserts @var{element} into @var{hash} and returns a null pointer.
-If the table already contains an element equal to @var{element}, returns
-the existing element without modifying @var{hash}.
+If the table already contains an element equal to @var{element}, it is
+returned without modifying @var{hash}.
@end deftypefun
@deftypefun {struct hash_elem *} hash_replace (struct hash *@var{hash}, struct hash_elem *@var{element})
equal to @var{element}.
The caller is responsible for deallocating any resources associated with
-the element returned, as appropriate. For example, if the hash table
+the returned element, as appropriate. For example, if the hash table
elements are dynamically allocated using @func{malloc}, then the caller
must @func{free} the element after it is no longer needed.
@end deftypefun
The element passed to the following functions is only used for hashing
and comparison purposes. It is never actually inserted into the hash
-table. Thus, only the key data in the element need be initialized, and
+table. Thus, only key data in the element needs to be initialized, and
other data in the element will not be used. It often makes sense to
declare an instance of the element type as a local variable, initialize
the key data, and then pass the address of its @struct{hash_elem} to
pointer is returned and @var{hash} is unchanged.
The caller is responsible for deallocating any resources associated with
-the element returned, as appropriate. For example, if the hash table
+the returned element, as appropriate. For example, if the hash table
elements are dynamically allocated using @func{malloc}, then the caller
must @func{free} the element after it is no longer needed.
@end deftypefun
scope:
@example
-/* @r{Returns the page containing the given virtual @var{address},
- or a null pointer if no such page exists.} */
+/* @r{Returns the page containing the given virtual @var{address},}
+ @r{or a null pointer if no such page exists.} */
struct page *
page_lookup (const void *address)
@{
it.)
@var{aux} is useful when you have some property of the data in the
-hash table that's both constant and needed for hashing or comparisons,
-but which is not stored in the data items themselves. For example, if
-the items in a hash table contain fixed-length strings, but the items
+hash table is both constant and needed for hashing or comparison,
+but not stored in the data items themselves. For example, if
+the items in a hash table are fixed-length strings, but the items
themselves don't indicate what that fixed length is, you could pass
the length as an @var{aux} parameter.
projects / pintos-anon / blobdiff