378 lines
15 KiB
Plaintext
378 lines
15 KiB
Plaintext
This file describes in little detail the modifications to the
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Objective-C runtime needed to make it thread safe.
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First off, kudos to Galen Hunt who is the author of this great work.
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If you have an comments or just want to know where to
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send me money to express your undying gratitude for threading the
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Objective-C runtime you can reach Galen at:
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gchunt@cs.rochester.edu
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Any questions, comments, bug reports, etc. should send email either to the
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GCC bug account or to:
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Scott Christley <scottc@net-community.com>
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* Sarray Threading:
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The most critical component of the Objective-C runtime is the sparse array
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structure (sarray). Sarrays store object selectors and implementations.
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Following in the tradition of the Objective-C runtime, my threading
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support assumes that fast message dispatching is far more important
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than *ANY* and *ALL* other operations. The message dispatching thus
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uses *NO* locks on any kind. In fact, if you look in sarray.h, you
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will notice that the message dispatching has not been modified.
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Instead, I have modified the sarray management functions so that all
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updates to the sarray data structure can be made in parallel will
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message dispatching.
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To support concurrent message dispatching, no dynamically allocated
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sarray data structures are freed while more than one thread is
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operational. Sarray data structures that are no longer in use are
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kept in a linked list of garbage and are released whenever the program
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is operating with a single thread. The programmer can also flush the
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garbage list by calling sarray_remove_garbage when the programmer can
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ensure that no message dispatching is taking place concurrently. The
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amount of un-reclaimed sarray garbage should normally be extremely
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small in a real program as sarray structures are freed only when using
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the "poseAs" functionality and early in program initialization, which
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normally occurs while the program is single threaded.
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******************************************************************************
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* Static Variables:
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The following variables are either statically or globally defined. This list
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does not include variables which are internal to implementation dependent
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versions of thread-*.c.
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The following threading designations are used:
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SAFE : Implicitly thread safe.
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SINGLE : Must only be used in single thread mode.
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MUTEX : Protected by single global mutex objc_runtime_mutex.
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UNUSED : Not used in the runtime.
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Variable Name: Usage: Defined: Also used in:
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=========================== ====== ============ =====================
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__objc_class_hash MUTEX class.c
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__objc_class_links_resolved UNUSED class.c runtime.h
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__objc_class_number MUTEX class.c
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__objc_dangling_categories UNUSED init.c
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__objc_module_list MUTEX init.c
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__objc_selector_array MUTEX selector.c
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__objc_selector_hash MUTEX selector.c
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__objc_selector_max_index MUTEX selector.c sendmsg.c runtime.h
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__objc_selector_names MUTEX selector.c
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__objc_thread_exit_status SAFE thread.c
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__objc_uninstalled_dtable MUTEX sendmsg.c selector.c
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_objc_load_callback SAFE init.c objc-api.h
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_objc_lookup_class SAFE class.c objc-api.h
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_objc_object_alloc SINGLE objects.c objc-api.h
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_objc_object_copy SINGLE objects.c objc-api.h
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_objc_object_dispose SINGLE objects.c objc-api.h
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frwd_sel SAFE2 sendmsg.c
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idxsize MUTEX sarray.c sendmsg.c sarray.h
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initialize_sel SAFE2 sendmsg.c
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narrays MUTEX sarray.c sendmsg.c sarray.h
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nbuckets MUTEX sarray.c sendmsg.c sarray.h
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nindices MUTEX sarray.c sarray.h
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previous_constructors SAFE1 init.c
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proto_class SAFE1 init.c
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unclaimed_categories MUTEX init.c
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unclaimed_proto_list MUTEX init.c
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uninitialized_statics MUTEX init.c
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Notes:
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1) Initialized once in unithread mode.
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2) Initialized value will always be same, guaranteed by lock on selector
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hash table.
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******************************************************************************
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* Frontend/Backend design:
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The design of the Objective-C runtime thread and mutex functions utilizes a
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frontend/backend implementation.
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The frontend, as characterized by the files thr.h and thr.c, is a set
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of platform independent structures and functions which represent the
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user interface. Objective-C programs should use these structures and
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functions for their thread and mutex work if they wish to maintain a
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high degree of portability across platforms.
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The backend is composed of a file with the necessary code to map the ObjC
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thread and mutex to a platform specific implementation. For example, the
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file thr-solaris.c contains the implementation for Solaris.
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If you are compiling libobjc as part of GCC, the thr-objc.c backend is
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always used; this backend uses GCC's gthread code. The thread system
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is automatically configured when GCC is configured. Important: make
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sure you configure GCC using `--enable-threads' if you want threads !
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If you want to compile libobjc standalone, then you would need to
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modify the configure.in and makefiles for it; and you need to pick an
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appropriate backend file for the target platform; you make this choice
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by assigning the OBJC_THREAD_FILE make variable to the basename of the
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backend file. For example, OBJC_THREAD_FILE=thr-posix would indicate
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that the generic posix backend file, thr-posix.c, should be compiled
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with the ObjC runtime library. If your platform does not support
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threads then you should specify the OBJC_THREAD_FILE=thr-single
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backend file to compile the ObjC runtime library without thread or
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mutex support; note that programs which rely upon the ObjC thread and
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mutex functions will compile and link correctly but attempting to
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create a thread or mutex will result in an error.
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It is questionable whether it is really necessary to have both a
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frontend and backend function for all available functionality. On the
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one hand, it provides a clear, consistent differentiation between what
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is public and what is private with the downside of having the overhead
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of multiple functions calls. For example, the function to have a
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thread yield the processor is objc_thread_yield; in the current
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implementation this produces a function call set:
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objc_thread_yield() -> __objc_thread_yield() -> system yield function
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This has two extra function calls over calling the platform specific function
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explicitly, but the issue is whether only the overhead of a single function
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is necessary.
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objc_thread_yield() -> system yield function
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This breaks the public/private dichotomy between the frontend/backend
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for the sake of efficiency. It is possible to just use a preprocessor
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define so as to eliminate the extra function call:
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#define objc_thread_yield() __objc_thread_yield()
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This has the undesirable effect that if objc_thread_yield is actually
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turned into a function based upon future need; then ObjC programs which
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access the thread functions would need to be recompiled versus just
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being relinked.
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******************************************************************************
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* Threads:
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The thread system attempts to create multiple threads using whatever
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operating system or library thread support is available. It does
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assume that all system functions are thread safe. Notably this means
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that the system implementation of malloc and free must be thread safe.
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If a system has multiple processors, the threads are configured for
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full parallel processing.
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* Backend initialization functions
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__objc_init_thread_system(void), int
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Initialize the thread subsystem. Called once by __objc_exec_class.
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Return -1 if error otherwise return 0.
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__objc_close_thread_system(void), int
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Closes the thread subsystem, not currently guaranteed to be called.
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Return -1 if error otherwise return 0.
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*****
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* Frontend thread functions
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* User programs should use these functions.
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objc_thread_detach(SEL selector, id object, id argument), objc_thread_t
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Creates and detaches a new thread. The new thread starts by
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sending the given selector with a single argument to the
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given object.
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objc_thread_set_priority(int priority), int
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Sets a thread's relative priority within the program. Valid
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options are:
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OBJC_THREAD_INTERACTIVE_PRIORITY
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OBJC_THREAD_BACKGROUND_PRIORITY
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OBJC_THREAD_LOW_PRIORITY
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objc_thread_get_priority(void), int
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Query a thread's priority.
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objc_thread_yield(void), void
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Yields processor to another thread with equal or higher
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priority. It is up to the system scheduler to determine if
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the processor is taken or not.
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objc_thread_exit(void), int
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Terminates a thread. If this is the last thread executing
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then the program will terminate.
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objc_thread_id(void), int
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Returns the current thread's id.
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objc_thread_set_data(void *value), int
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Set a pointer to the thread's local storage. Local storage is
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thread specific.
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objc_thread_get_data(void), void *
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Returns the pointer to the thread's local storage.
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*****
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* Backend thread functions
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* User programs should *NOT* directly call these functions.
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__objc_thread_detach(void (*func)(void *arg), void *arg), objc_thread_t
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Spawns a new thread executing func, called by objc_thread_detach.
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Return NULL if error otherwise return thread id.
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__objc_thread_set_priority(int priority), int
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Set the thread's priority, called by objc_thread_set_priority.
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Return -1 if error otherwise return 0.
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__objc_thread_get_priority(void), int
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Query a thread's priority, called by objc_thread_get_priority.
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Return -1 if error otherwise return the priority.
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__objc_thread_yield(void), void
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Yields the processor, called by objc_thread_yield.
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__objc_thread_exit(void), int
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Terminates the thread, called by objc_thread_exit.
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Return -1 if error otherwise function does not return.
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__objc_thread_id(void), objc_thread_t
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Returns the current thread's id, called by objc_thread_id.
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Return -1 if error otherwise return thread id.
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__objc_thread_set_data(void *value), int
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Set pointer for thread local storage, called by objc_thread_set_data.
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Returns -1 if error otherwise return 0.
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__objc_thread_get_data(void), void *
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Returns the pointer to the thread's local storage.
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Returns NULL if error, called by objc_thread_get_data.
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******************************************************************************
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* Mutexes:
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Mutexes can be locked recursively. Each locked mutex remembers
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its owner (by thread id) and how many times it has been locked. The
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last unlock on a mutex removes the system lock and allows other
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threads to access the mutex.
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*****
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* Frontend mutex functions
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* User programs should use these functions.
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objc_mutex_allocate(void), objc_mutex_t
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Allocates a new mutex. Mutex is initially unlocked.
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Return NULL if error otherwise return mutex pointer.
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objc_mutex_deallocate(objc_mutex_t mutex), int
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Free a mutex. Before freeing the mutex, makes sure that no
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one else is using it.
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Return -1 if error otherwise return 0.
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objc_mutex_lock(objc_mutex_t mutex), int
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Locks a mutex. As mentioned earlier, the same thread may call
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this routine repeatedly.
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Return -1 if error otherwise return 0.
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objc_mutex_trylock(objc_mutex_t mutex), int
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Attempts to lock a mutex. If lock on mutex can be acquired
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then function operates exactly as objc_mutex_lock.
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Return -1 if failed to acquire lock otherwise return 0.
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objc_mutex_unlock(objc_mutex_t mutex), int
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Unlocks the mutex by one level. Other threads may not acquire
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the mutex until this thread has released all locks on it.
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Return -1 if error otherwise return 0.
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*****
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* Backend mutex functions
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* User programs should *NOT* directly call these functions.
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__objc_mutex_allocate(objc_mutex_t mutex), int
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Allocates a new mutex, called by objc_mutex_allocate.
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Return -1 if error otherwise return 0.
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__objc_mutex_deallocate(objc_mutex_t mutex), int
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Free a mutex, called by objc_mutex_deallocate.
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Return -1 if error otherwise return 0.
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__objc_mutex_lock(objc_mutex_t mutex), int
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Locks a mutex, called by objc_mutex_lock.
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Return -1 if error otherwise return 0.
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__objc_mutex_trylock(objc_mutex_t mutex), int
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Attempts to lock a mutex, called by objc_mutex_trylock.
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Return -1 if failed to acquire lock or error otherwise return 0.
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__objc_mutex_unlock(objc_mutex_t mutex), int
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Unlocks the mutex, called by objc_mutex_unlock.
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Return -1 if error otherwise return 0.
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******************************************************************************
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* Condition Mutexes:
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Mutexes can be locked recursively. Each locked mutex remembers
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its owner (by thread id) and how many times it has been locked. The
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last unlock on a mutex removes the system lock and allows other
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threads to access the mutex.
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*
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* Frontend condition mutex functions
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* User programs should use these functions.
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*
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objc_condition_allocate(void), objc_condition_t
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Allocate a condition mutex.
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Return NULL if error otherwise return condition pointer.
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objc_condition_deallocate(objc_condition_t condition), int
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Deallocate a condition. Note that this includes an implicit
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condition_broadcast to insure that waiting threads have the
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opportunity to wake. It is legal to dealloc a condition only
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if no other thread is/will be using it. Does NOT check for
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other threads waiting but just wakes them up.
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Return -1 if error otherwise return 0.
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objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
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Wait on the condition unlocking the mutex until objc_condition_signal()
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or objc_condition_broadcast() are called for the same condition. The
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given mutex *must* have the depth 1 so that it can be unlocked
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here, for someone else can lock it and signal/broadcast the condition.
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The mutex is used to lock access to the shared data that make up the
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"condition" predicate.
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Return -1 if error otherwise return 0.
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objc_condition_broadcast(objc_condition_t condition), int
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Wake up all threads waiting on this condition. It is recommended that
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the called would lock the same mutex as the threads in
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objc_condition_wait before changing the "condition predicate"
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and make this call and unlock it right away after this call.
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Return -1 if error otherwise return 0.
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objc_condition_signal(objc_condition_t condition), int
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Wake up one thread waiting on this condition.
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Return -1 if error otherwise return 0.
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*
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* Backend condition mutex functions
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* User programs should *NOT* directly call these functions.
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*
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__objc_condition_allocate(objc_condition_t condition), int
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Allocate a condition mutex, called by objc_condition_allocate.
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Return -1 if error otherwise return 0.
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__objc_condition_deallocate(objc_condition_t condition), int
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Deallocate a condition, called by objc_condition_deallocate.
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Return -1 if error otherwise return 0.
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__objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
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Wait on the condition, called by objc_condition_wait.
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Return -1 if error otherwise return 0 when condition is met.
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__objc_condition_broadcast(objc_condition_t condition), int
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Wake up all threads waiting on this condition.
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Called by objc_condition_broadcast.
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Return -1 if error otherwise return 0.
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__objc_condition_signal(objc_condition_t condition), int
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Wake up one thread waiting on this condition.
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Called by objc_condition_signal.
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Return -1 if error otherwise return 0.
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