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460 lines
16 KiB
C
460 lines
16 KiB
C
/* GNU Objective C Runtime @synchronized implementation
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Copyright (C) 2010-2018 Free Software Foundation, Inc.
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Contributed by Nicola Pero <nicola.pero@meta-innovation.com>
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify it under the
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terms of the GNU General Public License as published by the Free Software
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Foundation; either version 3, or (at your option) any later version.
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GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
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details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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/* This file implements objc_sync_enter() and objc_sync_exit(), the
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two functions required to support @synchronized().
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objc_sync_enter(object) needs to get a recursive lock associated
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with 'object', and lock it.
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objc_sync_exit(object) needs to get the recursive lock associated
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with 'object', and unlock it. */
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/* To avoid the overhead of continuously allocating and deallocating
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locks, we implement a pool of locks. When a lock is needed for an
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object, we get a lock from the pool and associate it with the
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object.
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The lock pool need to be protected by its own lock (the
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"protection" lock), which has to be locked then unlocked each time
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objc_sync_enter() and objc_sync_exit() are called. To reduce the
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contention on the protection lock, instead of a single pool with a
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single (global) protection lock we use a number of smaller pools,
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each with its own pool protection lock. To decide which lock pool
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to use for each object, we compute a hash from the object pointer.
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The implementation of each lock pool uses a linked list of all the
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locks in the pool (both unlocked, and locked); this works in the
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assumption that the number of locks concurrently required is very
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low. In practice, it seems that you rarely see more than a few
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locks ever concurrently required.
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A standard case is a thread acquiring a lock recursively, over and
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over again: for example when most methods of a class are protected
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by @synchronized(self) but they also call each other. We use
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thread-local storage to implement a cache and optimize this case.
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The cache stores locks that the thread successfully acquired,
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allowing objc_sync_enter() and objc_sync_exit() to locate a lock
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which is already held by the current thread without having to use
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any protection lock or synchronization mechanism. It can so detect
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recursive locks/unlocks, and transform them into no-ops that
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require no actual locking or synchronization mechanisms at all. */
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/* You can disable the thread-local cache (most likely to benchmark
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the code with and without it) by compiling with
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-DSYNC_CACHE_DISABLE, or commenting out the following line. */
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/* #define SYNC_CACHE_DISABLE */
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/* If thread-local storage is not available, automatically disable the
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cache. */
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#ifndef HAVE_TLS
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# define SYNC_CACHE_DISABLE
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#endif
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#include "objc-private/common.h"
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#include "objc/objc-sync.h" /* For objc_sync_enter(), objc_sync_exit() */
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#include "objc/runtime.h" /* For objc_malloc() */
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#include "objc/thr.h" /* For objc_mutex_loc() and similar */
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#include "objc-private/objc-sync.h" /* For __objc_sync_init() */
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/* We have 32 pools of locks, each of them protected by its own
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protection lock. It's tempting to increase this number to reduce
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contention; but in our tests it is high enough. */
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#define SYNC_NUMBER_OF_POOLS 32
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/* Given an object, it determines which pool contains the associated
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lock. */
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#define SYNC_OBJECT_HASH(OBJECT) ((((size_t)OBJECT >> 8) ^ (size_t)OBJECT) & (SYNC_NUMBER_OF_POOLS - 1))
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/* The locks protecting each pool. */
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static objc_mutex_t sync_pool_protection_locks[SYNC_NUMBER_OF_POOLS];
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/* The data structure (linked list) holding the locks. */
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typedef struct lock_node
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{
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/* Pointer to next entry on the list. NULL indicates end of list.
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You need to hold the appropriate sync_pool_protection_locks[N] to
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read or write this variable. */
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struct lock_node *next;
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/* The (recursive) lock. Allocated when the node is created, and
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always not-NULL, and unchangeable, after that. */
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objc_mutex_t lock;
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/* This is how many times the objc_mutex_lock() has been called on
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the lock (it is 0 when the lock is unused). Used to track when
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the lock is no longer associated with an object and can be reused
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for another object. It records "real" locks, potentially (but
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not necessarily) by multiple threads. You need to hold the
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appropriate sync_pool_protection_locks[N] to read or write this
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variable. */
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unsigned int usage_count;
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/* The object that the lock is associated with. This variable can
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only be written when holding the sync_pool_protection_locks[N]
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and when node->usage_count == 0, ie, the lock is not being used.
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You can read this variable either when you hold the
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sync_pool_protection_locks[N] or when you hold node->lock,
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because in that case you know that node->usage_count can't get to
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zero until you release the lock. It is valid to have usage_count
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== 0 and object != nil; in that case, the lock is not currently
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being used, but is still currently associated with the
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object. */
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id object;
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/* This is a counter reserved for use by the thread currently
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holding the lock. So, you need to hold node->lock to read or
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write this variable. It is normally 0, and if the cache is not
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being used, it is kept at 0 (even if recursive locks are being
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done; in that case, no difference is made between recursive and
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non-recursive locks: they all increase usage_count, and call
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objc_mutex_lock()). When the cache is being used, a thread may
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be able to find a lock that it already holds using the cache; in
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that case, to perform additional locks/unlocks it can
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increase/decrease the recursive_usage_count (which does not
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require any synchronization with other threads, since it's
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protected by the node->lock itself) instead of the usage_count
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(which requires locking the pool protection lock). And it can
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skip the call to objc_mutex_lock/unlock too. */
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unsigned int recursive_usage_count;
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} *lock_node_ptr;
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/* The pools of locks. Each of them is a linked list of lock_nodes.
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In the list we keep both unlocked and locked nodes. */
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static lock_node_ptr sync_pool_array[SYNC_NUMBER_OF_POOLS];
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#ifndef SYNC_CACHE_DISABLE
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/* We store a cache of locks acquired by each thread in thread-local
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storage. */
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static __thread lock_node_ptr *lock_cache = NULL;
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/* This is a conservative implementation that uses a static array of
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fixed size as cache. Because the cache is an array that we scan
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linearly, the bigger it is, the slower it gets. This does not
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matter much at small sizes (eg, the overhead of checking 8 cache
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slots instead of 4 is very small compared to the other overheads
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involved such as function calls and lock/unlock operations), but at
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large sizes it becomes important as obviously there is a size over
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which using the cache backfires: the lookup is so slow that the
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cache slows down the software instead of speeding it up. In
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practice, it seems that most threads use a small number of
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concurrent locks, so we have a conservative implementation with a
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fixed-size cache of 8 locks which gives a very predictable
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behaviour. If a thread locks lots of different locks, only the
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first 8 get the speed benefits of the cache, but the cache remains
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always small, fast and predictable.
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SYNC_CACHE_SIZE is the size of the lock cache for each thread. */
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#define SYNC_CACHE_SIZE 8
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#endif /* SYNC_CACHE_DISABLE */
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/* Called at startup by init.c. */
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void
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__objc_sync_init (void)
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{
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int i;
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for (i = 0; i < SYNC_NUMBER_OF_POOLS; i++)
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{
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lock_node_ptr new_node;
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/* Create a protection lock for each pool. */
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sync_pool_protection_locks[i] = objc_mutex_allocate ();
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/* Preallocate a lock per pool. */
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new_node = objc_malloc (sizeof (struct lock_node));
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new_node->lock = objc_mutex_allocate ();
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new_node->object = nil;
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new_node->usage_count = 0;
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new_node->recursive_usage_count = 0;
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new_node->next = NULL;
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sync_pool_array[i] = new_node;
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}
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}
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int
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objc_sync_enter (id object)
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{
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#ifndef SYNC_CACHE_DISABLE
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int free_cache_slot;
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#endif
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int hash;
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lock_node_ptr node;
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lock_node_ptr unused_node;
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if (object == nil)
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return OBJC_SYNC_SUCCESS;
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#ifndef SYNC_CACHE_DISABLE
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if (lock_cache == NULL)
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{
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/* Note that this calloc only happen only once per thread, the
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very first time a thread does a objc_sync_enter(). */
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lock_cache = objc_calloc (SYNC_CACHE_SIZE, sizeof (lock_node_ptr));
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}
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/* Check the cache to see if we have a record of having already
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locked the lock corresponding to this object. While doing so,
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keep track of the first free cache node in case we need it
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later. */
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node = NULL;
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free_cache_slot = -1;
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{
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int i;
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for (i = 0; i < SYNC_CACHE_SIZE; i++)
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{
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lock_node_ptr locked_node = lock_cache[i];
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if (locked_node == NULL)
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{
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if (free_cache_slot == -1)
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free_cache_slot = i;
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}
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else if (locked_node->object == object)
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{
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node = locked_node;
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break;
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}
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}
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}
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if (node != NULL)
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{
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/* We found the lock. Increase recursive_usage_count, which is
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protected by node->lock, which we already hold. */
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node->recursive_usage_count++;
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/* There is no need to actually lock anything, since we already
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hold the lock. Correspondingly, objc_sync_exit() will just
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decrease recursive_usage_count and do nothing to unlock. */
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return OBJC_SYNC_SUCCESS;
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}
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#endif /* SYNC_CACHE_DISABLE */
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/* The following is the standard lookup for the lock in the standard
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pool lock. It requires a pool protection lock. */
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hash = SYNC_OBJECT_HASH(object);
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/* Search for an existing lock for 'object'. While searching, make
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note of any unused lock if we find any. */
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unused_node = NULL;
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objc_mutex_lock (sync_pool_protection_locks[hash]);
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node = sync_pool_array[hash];
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while (node != NULL)
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{
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if (node->object == object)
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{
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/* We found the lock. */
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node->usage_count++;
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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#ifndef SYNC_CACHE_DISABLE
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/* Put it in the cache. */
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if (free_cache_slot != -1)
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lock_cache[free_cache_slot] = node;
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#endif
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/* Lock it. */
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objc_mutex_lock (node->lock);
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return OBJC_SYNC_SUCCESS;
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}
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if (unused_node == NULL && node->usage_count == 0)
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{
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/* We found the first unused node. Record it. */
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unused_node = node;
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}
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node = node->next;
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}
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/* An existing lock for 'object' could not be found. */
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if (unused_node != NULL)
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{
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/* But we found a unused lock; use it. */
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unused_node->object = object;
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unused_node->usage_count = 1;
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unused_node->recursive_usage_count = 0;
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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#ifndef SYNC_CACHE_DISABLE
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if (free_cache_slot != -1)
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lock_cache[free_cache_slot] = unused_node;
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#endif
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objc_mutex_lock (unused_node->lock);
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return OBJC_SYNC_SUCCESS;
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}
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else
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{
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/* There are no unused nodes; allocate a new node. */
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lock_node_ptr new_node;
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/* Create the node. */
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new_node = objc_malloc (sizeof (struct lock_node));
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new_node->lock = objc_mutex_allocate ();
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new_node->object = object;
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new_node->usage_count = 1;
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new_node->recursive_usage_count = 0;
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/* Attach it at the beginning of the pool. */
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new_node->next = sync_pool_array[hash];
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sync_pool_array[hash] = new_node;
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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#ifndef SYNC_CACHE_DISABLE
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if (free_cache_slot != -1)
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lock_cache[free_cache_slot] = new_node;
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#endif
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objc_mutex_lock (new_node->lock);
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return OBJC_SYNC_SUCCESS;
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}
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}
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int
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objc_sync_exit (id object)
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{
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int hash;
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lock_node_ptr node;
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if (object == nil)
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return OBJC_SYNC_SUCCESS;
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#ifndef SYNC_CACHE_DISABLE
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if (lock_cache != NULL)
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{
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int i;
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/* Find the lock in the cache. */
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node = NULL;
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for (i = 0; i < SYNC_CACHE_SIZE; i++)
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{
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lock_node_ptr locked_node = lock_cache[i];
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if (locked_node != NULL && locked_node->object == object)
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{
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node = locked_node;
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break;
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}
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}
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/* Note that, if a node was found in the cache, the variable i
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now holds the index where it was found, which will be used to
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remove it from the cache. */
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if (node != NULL)
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{
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if (node->recursive_usage_count > 0)
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{
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node->recursive_usage_count--;
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return OBJC_SYNC_SUCCESS;
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}
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else
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{
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/* We need to do a real unlock. */
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hash = SYNC_OBJECT_HASH(object);
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/* TODO: If we had atomic increase/decrease operations
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with memory barriers, we could avoid the lock
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here! */
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objc_mutex_lock (sync_pool_protection_locks[hash]);
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node->usage_count--;
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/* Normally, we do not reset object to nil here. We'll
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leave the lock associated with that object, at zero
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usage count. This makes it slightly more efficient to
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provide a lock for that object if (as likely)
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requested again. If the object is deallocated, we
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don't care. It will never match a new lock that is
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requested, and the node will be reused at some point.
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But, if garbage collection is enabled, leaving a
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pointer to the object in memory might prevent the
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object from being released. In that case, we remove
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it (TODO: maybe we should avoid using the garbage
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collector at all ? Nothing is ever deallocated in
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this file). */
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#if OBJC_WITH_GC
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node->object = nil;
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#endif
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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/* PS: Between objc_mutex_unlock
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(sync_pool_protection_locks[hash]) and
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objc_mutex_unlock (node->lock), the pool is unlocked
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so other threads may allocate this same lock to
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another object (!). This is not a problem, but it is
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curious. */
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objc_mutex_unlock (node->lock);
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/* Remove the node from the cache. */
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lock_cache[i] = NULL;
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return OBJC_SYNC_SUCCESS;
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}
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}
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}
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#endif
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/* The cache either wasn't there, or didn't work (eg, we overflowed
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it at some point and stopped recording new locks in the cache).
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Proceed with a full search of the lock pool. */
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hash = SYNC_OBJECT_HASH(object);
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objc_mutex_lock (sync_pool_protection_locks[hash]);
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/* Search for an existing lock for 'object'. */
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node = sync_pool_array[hash];
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while (node != NULL)
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{
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if (node->object == object)
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{
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/* We found the lock. */
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node->usage_count--;
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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objc_mutex_unlock (node->lock);
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/* No need to remove the node from the cache, since it
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wasn't found in the cache when we looked for it! */
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return OBJC_SYNC_SUCCESS;
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}
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node = node->next;
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}
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objc_mutex_unlock (sync_pool_protection_locks[hash]);
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/* A lock for 'object' to unlock could not be found (!!). */
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return OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
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}
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