X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2Freference.texi;h=463f728fcdc908ecd65f5f75d117645daee5955d;hb=3aed0d2d4a90ee50db206d7442c4a87207f59d4d;hp=3fe8b68a19d55c08901bef173bc144890450257b;hpb=95e1f45529540b68ac09b2e54ce4fe85d7302d96;p=pintos-anon diff --git a/doc/reference.texi b/doc/reference.texi index 3fe8b68..463f728 100644 --- a/doc/reference.texi +++ b/doc/reference.texi @@ -163,7 +163,7 @@ this, but calling it a second time is harmless. The next block of functions we call initialize 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 (@xpref{Page Allocator}). +doles out memory one or more pages at a time (@pxref{Page Allocator}). @func{malloc_init} sets up the allocator that handles allocations of arbitrary-size blocks of memory (@pxref{Block Allocator}). @@ -230,27 +230,27 @@ grows downward from the end of the page. It looks like this: @example @group - 4 kB +---------------------------------+ - | kernel stack | - | | | - | | | - | V | - | grows downward | - | | - | | - | | - | | - | | - | | - | | - | | - +---------------------------------+ - | magic | - | : | - | : | - | status | - | tid | - 0 kB +---------------------------------+ + 4 kB +---------------------------------+ + | kernel stack | + | | | + | | | + | V | + | grows downward | + | | + | | + | | + | | + | | + | | + | | + | | +sizeof (struct thread) +---------------------------------+ + | magic | + | : | + | : | + | status | + | tid | + 0 kB +---------------------------------+ @end group @end example @@ -276,6 +276,7 @@ if you like. @end deftypecv @deftypecv {Member} {@struct{thread}} {enum thread_status} status +@anchor{Thread States} The thread's state, one of the following: @defvr {Thread State} @code{THREAD_RUNNING} @@ -294,7 +295,12 @@ invoked. Ready threads are kept in a doubly linked list called The thread is waiting for something, e.g.@: a lock to become available, an interrupt to be invoked. The thread won't be scheduled again until it transitions to the @code{THREAD_READY} state with a -call to @func{thread_unblock}. +call to @func{thread_unblock}. This is most conveniently done +indirectly, using one of the Pintos synchronization primitives that +block and unblock threads automatically (@pxref{Synchronization}). + +There is no @i{a priori} way to tell what a blocked thread is waiting +for, but a backtrace can help (@pxref{Backtraces}). @end defvr @defvr {Thread State} @code{THREAD_DYING} @@ -332,23 +338,24 @@ priority scheduling in project 1 (@pxref{Priority Scheduling}). @deftypecv {Member} {@struct{thread}} {@struct{list_elem}} elem A ``list element'' used to put the thread into doubly linked lists, -either the list of threads ready to run or a list of threads waiting -on a semaphore. Take a look at @file{lib/kernel/list.h} for -information on how to use Pintos doubly linked lists. +either @code{ready_list} (the list of threads ready to run) or a list of +threads waiting on a semaphore in @func{sema_down}. It can do double +duty because a thread waiting on a semaphore is not ready, and vice +versa. @end deftypecv @deftypecv {Member} {@struct{thread}} {uint32_t *} pagedir -Only present in project 2 and later. +Only present in project 2 and later. @xref{Page Tables}. @end deftypecv @deftypecv {Member} {@struct{thread}} {unsigned} magic -Always set to @code{THREAD_MAGIC}, which is just a random number defined +Always set to @code{THREAD_MAGIC}, which is just an arbitrary number defined in @file{threads/thread.c}, and used to detect stack overflow. @func{thread_current} checks that the @code{magic} member of the running thread's @struct{thread} is set to @code{THREAD_MAGIC}. Stack overflow -will normally change this value, triggering the assertion. For greatest -benefit, as you add members to @struct{thread}, leave @code{magic} as -the final member. +tends to change this value, triggering the assertion. For greatest +benefit, as you add members to @struct{thread}, leave @code{magic} at +the end. @end deftypecv @node Thread Functions @@ -396,20 +403,21 @@ Creates and starts a new thread named @var{name} with the given @func{thread_create} allocates a page for the thread's @struct{thread} and stack and initializes its members, then it sets -up a set of fake stack frames for it (more about this -later). The thread is initialized in the blocked state, so the final -action before returning is to unblock it, which allows the new thread to -be scheduled. -@end deftypefun +up a set of fake stack frames for it (@pxref{Thread Switching}). The +thread is initialized in the blocked state, then unblocked just before +returning, which allows the new thread to +be scheduled (@pxref{Thread States}). @deftp {Type} {void thread_func (void *@var{aux})} -This is the type of a thread function. Its @var{aux} argument is the -value passed to @func{thread_create}. +This is the type of the function passed to @func{thread_create}, whose +@var{aux} argument is passed along as the function's argument. @end deftp +@end deftypefun @deftypefun void thread_block (void) Transitions the running thread from the running state to the blocked -state. The thread will not run again until @func{thread_unblock} is +state (@pxref{Thread States}). The thread will not run again until +@func{thread_unblock} is called on it, so you'd better have some way arranged for that to happen. Because @func{thread_block} is so low-level, you should prefer to use one of the synchronization primitives instead (@pxref{Synchronization}). @@ -417,8 +425,9 @@ one of the synchronization primitives instead (@pxref{Synchronization}). @deftypefun void thread_unblock (struct thread *@var{thread}) Transitions @var{thread}, which must be in the blocked state, to the -ready state, allowing it to resume running. This is called when the -event that the thread is waiting for occurs, e.g.@: when the lock that +ready state, allowing it to resume running (@pxref{Thread States}). +This is called when the event that the thread is waiting for occurs, +e.g.@: when the lock that the thread is waiting on becomes available. @end deftypefun @@ -450,20 +459,20 @@ time. @deftypefun int thread_get_priority (void) @deftypefunx void thread_set_priority (int @var{new_priority}) -Skeleton to set and get thread priority. @xref{Priority Scheduling}. +Stub to set and get thread priority. @xref{Priority Scheduling}. @end deftypefun @deftypefun int thread_get_nice (void) @deftypefunx void thread_set_nice (int @var{new_nice}) @deftypefunx int thread_get_recent_cpu (void) @deftypefunx int thread_get_load_avg (void) -Skeletons for the advanced scheduler. @xref{4.4BSD Scheduler}. +Stubs for the advanced scheduler. @xref{4.4BSD Scheduler}. @end deftypefun @node Thread Switching @subsection Thread Switching -@func{schedule} is the function responsible for switching threads. It +@func{schedule} is responsible for switching threads. It is internal to @file{threads/thread.c} and called only by the three public thread functions that need to switch threads: @func{thread_block}, @func{thread_exit}, and @func{thread_yield}. @@ -471,7 +480,7 @@ Before any of these functions call @func{schedule}, they disable interrupts (or ensure that they are already disabled) and then change the running thread's state to something other than running. -@func{schedule} is simple but tricky. It records the +@func{schedule} is short but tricky. It records the current thread in local variable @var{cur}, determines the next thread to run as local variable @var{next} (by calling @func{next_thread_to_run}), and then calls @func{switch_threads} to do @@ -486,7 +495,7 @@ CPU's current stack pointer in the current @struct{thread}'s @code{stack} member, restores the new thread's @code{stack} into the CPU's stack pointer, restores registers from the stack, and returns. -The rest of the scheduler is implemented as @func{schedule_tail}. It +The rest of the scheduler is implemented in @func{schedule_tail}. It marks the new thread as running. If the thread we just switched from is in the dying state, then it also frees the page that contained the dying thread's @struct{thread} and stack. These couldn't be freed @@ -494,7 +503,7 @@ prior to the thread switch because the switch needed to use it. Running a thread for the first time is a special case. When @func{thread_create} creates a new thread, it goes through a fair -amount of trouble to get it started properly. In particular, a new +amount of trouble to get it started properly. In particular, the new thread hasn't started running yet, so there's no way for it to be running inside @func{switch_threads} as the scheduler expects. To solve the problem, @func{thread_create} creates some fake stack frames @@ -531,7 +540,7 @@ interrupts and calls the thread's function (the function passed to @section Synchronization If sharing of resources between threads is not handled in a careful, -controlled fashion, then the result is usually a big mess. +controlled fashion, the result is usually a big mess. This is especially the case in operating system kernels, where faulty sharing can crash the entire machine. Pintos provides several synchronization primitives to help out. @@ -540,7 +549,7 @@ synchronization primitives to help out. * Disabling Interrupts:: * Semaphores:: * Locks:: -* Condition Variables:: +* Monitors:: * Memory Barriers:: @end menu @@ -598,12 +607,8 @@ Turns interrupts off. Returns the previous interrupt state. @node Semaphores @subsection Semaphores -Pintos' semaphore type and operations are declared in -@file{threads/synch.h}. - -@deftp {Type} {struct semaphore} -Represents a @dfn{semaphore}, a nonnegative integer together with two -operators that manipulate it atomically, which are: +A @dfn{semaphore} is a nonnegative integer together with two operators +that manipulate it atomically, which are: @itemize @bullet @item @@ -632,6 +637,15 @@ more appropriate. Semaphores can also be initialized to values larger than 1. These are rarely used. + +Semaphores were invented by Edsger Dijkstra and first used in the THE +operating system (@bibref{THE}). + +Pintos' semaphore type and operations are declared in +@file{threads/synch.h}. + +@deftp {Type} {struct semaphore} +Represents a semaphore. @end deftp @deftypefun void sema_init (struct semaphore *@var{sema}, unsigned @var{value}) @@ -646,17 +660,22 @@ its value to become positive and then decrementing it by one. @deftypefun bool sema_try_down (struct semaphore *@var{sema}) Tries to execute the ``down'' or ``P'' operation on @var{sema}, -without waiting. Returns true if @var{sema} had a positive value -that was successfully decremented, or false if it was already -zero and thus could not be decremented. Calling this function in a -tight loop wastes CPU time (use @func{sema_down} instead, or find a -different approach). +without waiting. Returns true if @var{sema} +was successfully decremented, or false if it was already +zero and thus could not be decremented without waiting. Calling this +function in a +tight loop wastes CPU time, so use @func{sema_down} or find a +different approach instead. @end deftypefun @deftypefun void sema_up (struct semaphore *@var{sema}) Executes the ``up'' or ``V'' operation on @var{sema}, incrementing its value. If any threads are waiting on @var{sema}, wakes one of them up. + +Unlike most synchronization primitives, @func{sema_up} may be called +inside an external interrupt handler (@pxref{External Interrupt +Handling}). @end deftypefun Semaphores are internally built out of disabling interrupt @@ -668,29 +687,31 @@ implementation in @file{lib/kernel/list.c}. @node Locks @subsection Locks -Lock types and functions are declared in @file{threads/synch.h}. +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''. -@deftp {Type} {struct lock} -Represents a @dfn{lock}, a specialized semaphore with an initial value -of 1 (@pxref{Semaphores}). The difference between a lock and such a -semaphore is twofold. First, a semaphore does not have an owner, -meaning that one thread can ``down'' the semaphore and then another one -``up'' it, but a single thread must both acquire and release a lock. -Second, a semaphore can have a value greater than 1, but a lock can only -be owned by a single thread at a time. If these restrictions prove -onerous, it's a good sign that a semaphore should be used, instead of a -lock. +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 +release it. If this restriction is a problem, it's a good sign that a +semaphore should be used, instead of a lock. Locks in Pintos are not ``recursive,'' that is, it is an error for the thread currently holding a lock to try to acquire that lock. + +Lock types and functions are declared in @file{threads/synch.h}. + +@deftp {Type} {struct lock} +Represents a lock. @end deftp @deftypefun void lock_init (struct lock *@var{lock}) Initializes @var{lock} as a new lock. +The lock is not initially owned by any thread. @end deftypefun @deftypefun void lock_acquire (struct lock *@var{lock}) -Acquires @var{lock} for use by the current thread, first waiting for +Acquires @var{lock} for the current thread, first waiting for any current owner to release it if necessary. @end deftypefun @@ -698,7 +719,7 @@ any current owner to release it if necessary. Tries to acquire @var{lock} for use by the current thread, without waiting. Returns true if successful, false if the lock is already owned. Calling this function in a tight loop is a bad idea because it -wastes CPU time (use @func{lock_acquire} instead). +wastes CPU time, so use @func{lock_acquire} instead. @end deftypefun @deftypefun void lock_release (struct lock *@var{lock}) @@ -708,37 +729,41 @@ Releases @var{lock}, which the current thread must own. @deftypefun bool lock_held_by_current_thread (const struct lock *@var{lock}) Returns true if the running thread owns @var{lock}, false otherwise. -@end deftypefun - -@node Condition Variables -@subsection Condition Variables +There is no function to test whether an arbitrary thread owns a lock, +because the answer could change before the caller could act on it. +@end deftypefun + +@node Monitors +@subsection Monitors + +A @dfn{monitor} is a higher-level form of synchronization than a +semaphore or a lock. A monitor consists of data being synchronized, +plus a lock, called the @dfn{monitor lock}, and one or more +@dfn{condition variables}. Before it accesses the protected data, a +thread first acquires the monitor lock. It is then said to be ``in the +monitor''. While in the monitor, the thread has control over all the +protected data, which it may freely examine or modify. When access to +the protected data is complete, it releases the monitor lock. + +Condition variables allow code in the monitor to wait for a condition to +become true. Each condition variable is associated with an abstract +condition, e.g.@: ``some data has arrived for processing'' or ``over 10 +seconds has passed since the user's last keystroke''. When code in the +monitor needs to wait for a condition to become true, it ``waits'' on +the associated condition variable, which releases the lock and waits for +the condition to be signaled. If, on the other hand, it has caused one +of these conditions to become true, it ``signals'' the condition to wake +up one waiter, or ``broadcasts'' the condition to wake all of them. + +The theoretical framework for monitors was laid out by C.@: A.@: R.@: +Hoare (@bibref{Hoare}). Their practical usage was later elaborated in a +paper on the Mesa operating system (@bibref{Mesa}). Condition variable types and functions are declared in @file{threads/synch.h}. @deftp {Type} {struct condition} -Represents a condition variable, which allows one piece of code to -signal a condition -and cooperating code to receive the signal and act upon it. Each -condition variable is associated with a lock. A given condition -variable is associated with only a single lock, but one lock may be -associated with any number of condition variables. A set of condition -variables taken together with their lock is called a ``monitor.'' - -A thread that owns the monitor lock is said to be ``in the monitor.'' -The thread in the monitor has control over all the data protected by -the lock. It may freely examine or modify this data. If it discovers -that it needs to wait for some condition to become true, then it -``waits'' on the associated condition, which releases the lock and -waits for the condition to be signaled. If, on the other hand, it has -caused one of these conditions to become true, it ``signals'' the -condition to wake up one waiter, or ``broadcasts'' the condition to -wake all of them. - -Pintos monitors are ``Mesa'' style, not -``Hoare'' style. That is, sending and receiving a signal are not an -atomic operation. Thus, typically the caller must recheck the -condition after the wait completes and, if necessary, wait again. +Represents a condition variable. @end deftp @deftypefun void cond_init (struct condition *@var{cond}) @@ -750,6 +775,11 @@ Atomically releases @var{lock} (the monitor lock) and waits for @var{cond} to be signaled by some other piece of code. After @var{cond} is signaled, reacquires @var{lock} before returning. @var{lock} must be held before calling this function. + +Sending a signal and waking up from a wait are not an atomic operation. +Thus, typically @func{cond_wait}'s caller must recheck the condition +after the wait completes and, if necessary, wait again. See the next +section for an example. @end deftypefun @deftypefun void cond_signal (struct condition *@var{cond}, struct lock *@var{lock}) @@ -768,8 +798,9 @@ function. @subsubsection Monitor Example The classical example of a monitor is handling a buffer into which one -``producer'' thread writes characters and out of which a second -``consumer'' thread reads characters. To implement this case we need, +or more +``producer'' threads write characters and out of which one or more +``consumer'' threads read characters. To implement this we need, besides the monitor lock, two condition variables which we will call @var{not_full} and @var{not_empty}: @@ -809,6 +840,9 @@ char get (void) @{ @node Memory Barriers @subsection Memory 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