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* svn_named_atomic.c: routines for machine-wide named atomics.
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* ====================================================================
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* Licensed to the Apache Software Foundation (ASF) under one
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* or more contributor license agreements. See the NOTICE file
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* distributed with this work for additional information
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* regarding copyright ownership. The ASF licenses this file
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* to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing,
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* software distributed under the License is distributed on an
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* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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* KIND, either express or implied. See the License for the
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* specific language governing permissions and limitations
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* ====================================================================
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#include "private/svn_named_atomic.h"
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#include <apr_global_mutex.h>
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#include "svn_private_config.h"
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#include "private/svn_atomic.h"
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#include "private/svn_mutex.h"
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#include "svn_pools.h"
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#include "svn_dirent_uri.h"
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/* Implementation aspects.
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* We use a single shared memory block (memory mapped file) that will be
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* created by the first user and merely mapped by all subsequent ones.
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* The memory block contains an short header followed by a fixed-capacity
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* array of named atomics. The number of entries currently in use is stored
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* Finding / creating the MMAP object as well as adding new array entries
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* is being guarded by an APR global mutex. Since releasing the MMAP
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* structure and closing the underlying does not affect other users of the
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* same, cleanup will not be synchronized.
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* The array is append-only. Once a process mapped the block into its
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* address space, it may freely access any of the used entries. However,
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* it must synchronize access to the volatile data within the entries.
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* On Windows and where otherwise supported by GCC, lightweight "lock-free"
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* synchronization will be used. Other targets serialize all access using
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* Atomics will be identified by their name (a short string) and lookup
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* takes linear time. But even that takes only about 10 microseconds for a
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* full array scan -- which is in the same order of magnitude than e.g. a
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* single global mutex lock / unlock pair.
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/* Capacity of our shared memory object, i.e. max number of named atomics
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* that may be created. Should have the form 2**N - 1.
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#define MAX_ATOMIC_COUNT 1023
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/* We choose the size of a single named atomic object to fill a complete
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* cache line (on most architectures). Thereby, we minimize the cache
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* sync. overhead between different CPU cores.
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#define CACHE_LINE_LENGTH 64
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/* We need 8 bytes for the actual value and the remainder is used to
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* store the NUL-terminated name.
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* Must not be smaller than SVN_NAMED_ATOMIC__MAX_NAME_LENGTH.
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#define MAX_NAME_LENGTH (CACHE_LINE_LENGTH - sizeof(apr_int64_t) - 1)
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/* Particle that will be appended to the namespace name to form the
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* name of the mutex / lock file used for that namespace.
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#define MUTEX_NAME_SUFFIX ".mutex"
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/* Particle that will be appended to the namespace name to form the
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* name of the shared memory file that backs that namespace.
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#define SHM_NAME_SUFFIX ".shm"
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/* Platform-dependent implementations of our basic atomic operations.
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* NA_SYNCHRONIZE(op) will ensure that the OP gets executed atomically.
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* This will be zero-overhead if OP itself is already atomic.
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* (We don't call it SYNCHRONIZE because Windows has a preprocess macro by
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* The default implementation will use the same mutex for initialization
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* as well as any type of data access. This is quite expensive and we
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* can do much better on most platforms.
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#if defined(WIN32) && ((_WIN32_WINNT >= 0x0502) || defined(InterlockedExchangeAdd64))
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/* Interlocked API / intrinsics guarantee full data synchronization
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#define synched_read(mem) *mem
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#define synched_write(mem, value) InterlockedExchange64(mem, value)
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#define synched_add(mem, delta) InterlockedExchangeAdd64(mem, delta)
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#define synched_cmpxchg(mem, value, comperand) \
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InterlockedCompareExchange64(mem, value, comperand)
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#define NA_SYNCHRONIZE(_atomic,op) op;
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#define NA_SYNCHRONIZE_IS_FAST TRUE
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#elif SVN_HAS_ATOMIC_BUILTINS
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/* GCC provides atomic intrinsics for most common CPU types
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#define synched_read(mem) *mem
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#define synched_write(mem, value) __sync_lock_test_and_set(mem, value)
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#define synched_add(mem, delta) __sync_add_and_fetch(mem, delta)
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#define synched_cmpxchg(mem, value, comperand) \
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__sync_val_compare_and_swap(mem, comperand, value)
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#define NA_SYNCHRONIZE(_atomic,op) op;
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#define NA_SYNCHRONIZE_IS_FAST TRUE
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/* Default implementation
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synched_read(volatile apr_int64_t *mem)
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synched_write(volatile apr_int64_t *mem, apr_int64_t value)
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apr_int64_t old_value = *mem;
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synched_add(volatile apr_int64_t *mem, apr_int64_t delta)
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return *mem += delta;
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synched_cmpxchg(volatile apr_int64_t *mem,
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apr_int64_t comperand)
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apr_int64_t old_value = *mem;
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if (old_value == comperand)
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#define NA_SYNCHRONIZE(_atomic,op)\
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SVN_ERR(lock(_atomic->mutex));\
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SVN_ERR(unlock(_atomic->mutex,SVN_NO_ERROR));\
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#define NA_SYNCHRONIZE_IS_FAST FALSE
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/* Structure describing a single atomic: its VALUE and NAME.
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struct named_atomic_data_t
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volatile apr_int64_t value;
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char name[MAX_NAME_LENGTH + 1];
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/* Content of our shared memory buffer. COUNT is the number
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* of used entries in ATOMICS. Insertion is append-only.
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* PADDING is used to align the header information with the
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* atomics to create a favorable data alignment.
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volatile apr_uint32_t count;
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char padding [sizeof(struct named_atomic_data_t) - sizeof(apr_uint32_t)];
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struct named_atomic_data_t atomics[MAX_ATOMIC_COUNT];
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/* Structure combining all objects that we need for access serialization.
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/* Inter-process sync. is handled by through lock file. */
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apr_file_t *lock_file;
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/* Pool to be used with lock / unlock functions */
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/* API structure combining the atomic data and the access mutex
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struct svn_named_atomic__t
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/* pointer into the shared memory */
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struct named_atomic_data_t *data;
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/* sync. object; never NULL (even if unused) */
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struct mutex_t *mutex;
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/* This is intended to be a singleton struct. It contains all
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* information necessary to initialize and access the shared
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struct svn_atomic_namespace__t
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/* Pointer to the shared data mapped into our process */
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struct shared_data_t *data;
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/* Last time we checked, this was the number of used
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* (i.e. fully initialized) items. I.e. we can read
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* their names without further sync. */
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volatile svn_atomic_t min_used;
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/* for each atomic in the shared memory, we hand out
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* at most one API-level object. */
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struct svn_named_atomic__t atomics[MAX_ATOMIC_COUNT];
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/* Synchronization object for this namespace */
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struct mutex_t mutex;
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/* On most operating systems APR implements file locks per process, not
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* per file. I.e. the lock file will only sync. among processes but within
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* a process, we must use a mutex to sync the threads. */
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/* Compare ../libsvn_fs_fs/fs.h:SVN_FS_FS__USE_LOCK_MUTEX */
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#if APR_HAS_THREADS && !defined(WIN32)
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#define USE_THREAD_MUTEX 1
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#define USE_THREAD_MUTEX 0
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/* Used for process-local thread sync.
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static svn_mutex__t *thread_mutex = NULL;
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/* Initialization flag for the above used by svn_atomic__init_once.
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static volatile svn_atomic_t mutex_initialized = FALSE;
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/* Initialize the thread sync. structures.
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* To be called by svn_atomic__init_once.
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init_thread_mutex(void *baton, apr_pool_t *pool)
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/* let the mutex live as long as the APR */
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apr_pool_t *global_pool = svn_pool_create(NULL);
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return svn_mutex__init(&thread_mutex, USE_THREAD_MUTEX, global_pool);
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#endif /* APR_HAS_MMAP */
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/* Utility that acquires our global mutex and converts error types.
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lock(struct mutex_t *mutex)
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/* Get lock on the filehandle. */
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SVN_ERR(svn_mutex__lock(thread_mutex));
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err = svn_io_lock_open_file(mutex->lock_file, TRUE, FALSE, mutex->pool);
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? svn_mutex__unlock(thread_mutex, err)
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/* Utility that releases the lock previously acquired via lock(). If the
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* unlock succeeds and OUTER_ERR is not NULL, OUTER_ERR will be returned.
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* Otherwise, return the result of the unlock operation.
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unlock(struct mutex_t *mutex, svn_error_t * outer_err)
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svn_error_t *unlock_err
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= svn_io_unlock_open_file(mutex->lock_file, mutex->pool);
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return svn_mutex__unlock(thread_mutex,
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svn_error_compose_create(outer_err,
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/* The last user to close a particular namespace should also remove the
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* lock file. Failure to do so, however, does not affect further uses
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* of the same namespace.
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delete_lock_file(void *arg)
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struct mutex_t *mutex = arg;
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const char *lock_name = NULL;
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/* locks have already been cleaned up. Simply close the file */
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apr_status_t status = apr_file_close(mutex->lock_file);
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/* Remove the file from disk. This will fail if there ares still other
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* users of this lock file, i.e. namespace. */
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apr_file_name_get(&lock_name, mutex->lock_file);
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apr_file_remove(lock_name, mutex->pool);
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#endif /* APR_HAS_MMAP */
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/* Validate the ATOMIC parameter, i.e it's address. Correct code will
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* never need this but if someone should accidentally to use a NULL or
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* incomplete structure, let's catch that here instead of segfaulting.
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validate(svn_named_atomic__t *atomic)
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return atomic && atomic->data && atomic->mutex
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: svn_error_create(SVN_ERR_BAD_ATOMIC, 0, _("Not a valid atomic"));
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/* Auto-initialize and return in *ATOMIC the API-level object for the
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* atomic with index I within NS. */
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return_atomic(svn_named_atomic__t **atomic,
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svn_atomic_namespace__t *ns,
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*atomic = &ns->atomics[i];
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if (ns->atomics[i].data == NULL)
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(*atomic)->mutex = &ns->mutex;
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(*atomic)->data = &ns->data->atomics[i];
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svn_named_atomic__is_supported(void)
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#elif !defined(_WIN32)
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static svn_tristate_t result = svn_tristate_unknown;
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if (result == svn_tristate_unknown)
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/* APR SHM implementation requires the creation of global objects */
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HANDLE handle = CreateFileMappingA(INVALID_HANDLE_VALUE,
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"Global\\__RandomXZY_svn");
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result = svn_tristate_true;
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result = svn_tristate_false;
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return result == svn_tristate_true;
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svn_named_atomic__is_efficient(void)
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return NA_SYNCHRONIZE_IS_FAST;
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svn_atomic_namespace__create(svn_atomic_namespace__t **ns,
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apr_pool_t *result_pool)
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return svn_error_create(APR_ENOTIMPL, NULL, NULL);
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apr_status_t apr_err;
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const char *shm_name, *lock_name;
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apr_pool_t *subpool = svn_pool_create(result_pool);
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/* allocate the namespace data structure
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svn_atomic_namespace__t *new_ns = apr_pcalloc(result_pool, sizeof(**ns));
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/* construct the names of the system objects that we need
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shm_name = apr_pstrcat(subpool, name, SHM_NAME_SUFFIX, NULL);
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lock_name = apr_pstrcat(subpool, name, MUTEX_NAME_SUFFIX, NULL);
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/* initialize the lock objects
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SVN_ERR(svn_atomic__init_once(&mutex_initialized, init_thread_mutex, NULL,
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new_ns->mutex.pool = result_pool;
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SVN_ERR(svn_io_file_open(&new_ns->mutex.lock_file, lock_name,
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APR_READ | APR_WRITE | APR_CREATE,
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/* Make sure the last user of our lock file will actually remove it.
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* Please note that only the last file handle begin closed will actually
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* remove the underlying file (see docstring for apr_file_remove).
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apr_pool_cleanup_register(result_pool, &new_ns->mutex,
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apr_pool_cleanup_null);
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/* Prevent concurrent initialization.
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SVN_ERR(lock(&new_ns->mutex));
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/* First, make sure that the underlying file exists. If it doesn't
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* exist, create one and initialize its content.
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err = svn_io_file_open(&file, shm_name,
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APR_READ | APR_WRITE | APR_CREATE,
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err = svn_io_stat(&finfo, shm_name, APR_FINFO_SIZE, subpool);
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if (!err && finfo.size < sizeof(struct shared_data_t))
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/* Zero all counters, values and names.
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struct shared_data_t initial_data;
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memset(&initial_data, 0, sizeof(initial_data));
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err = svn_io_file_write_full(file, &initial_data,
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sizeof(initial_data), NULL,
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/* Now, map it into memory.
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apr_err = apr_mmap_create(&mmap, file, 0, sizeof(*new_ns->data),
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APR_MMAP_READ | APR_MMAP_WRITE , result_pool);
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new_ns->data = mmap->mm;
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err = svn_error_createf(apr_err, NULL,
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_("MMAP failed for file '%s'"), shm_name);
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svn_pool_destroy(subpool);
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if (!err && new_ns->data)
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/* Detect severe cases of corruption (i.e. when some outsider messed
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* with our data file)
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if (new_ns->data->count > MAX_ATOMIC_COUNT)
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return svn_error_create(SVN_ERR_CORRUPTED_ATOMIC_STORAGE, 0,
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_("Number of atomics in namespace is too large."));
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/* Cache the number of existing, complete entries. There can't be
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* incomplete ones from other processes because we hold the mutex.
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* Our process will also not access this information since we are
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* either being called from within svn_atomic__init_once or by
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* svn_atomic_namespace__create for a new object.
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new_ns->min_used = new_ns->data->count;
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/* Unlock to allow other processes may access the shared memory as well.
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return unlock(&new_ns->mutex, err);
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#endif /* APR_HAS_MMAP */
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svn_atomic_namespace__cleanup(const char *name,
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const char *shm_name, *lock_name;
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/* file names used for the specified namespace */
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shm_name = apr_pstrcat(pool, name, SHM_NAME_SUFFIX, NULL);
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lock_name = apr_pstrcat(pool, name, MUTEX_NAME_SUFFIX, NULL);
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/* remove these files if they exist */
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SVN_ERR(svn_io_remove_file2(shm_name, TRUE, pool));
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SVN_ERR(svn_io_remove_file2(lock_name, TRUE, pool));
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svn_named_atomic__get(svn_named_atomic__t **atomic,
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svn_atomic_namespace__t *ns,
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svn_boolean_t auto_create)
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apr_uint32_t i, count;
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svn_error_t *error = SVN_NO_ERROR;
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apr_size_t len = strlen(name);
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/* Check parameters and make sure we return a NULL atomic
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* in case of failure.
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if (len > SVN_NAMED_ATOMIC__MAX_NAME_LENGTH)
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return svn_error_create(SVN_ERR_BAD_ATOMIC, 0,
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_("Atomic's name is too long."));
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/* If no namespace has been provided, bail out.
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if (ns == NULL || ns->data == NULL)
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return svn_error_create(SVN_ERR_BAD_ATOMIC, 0,
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_("Namespace has not been initialized."));
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/* Optimistic lookup.
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* Because we never change the name of existing atomics and may only
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* append new ones, we can safely compare the name of existing ones
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* with the name that we are looking for.
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for (i = 0, count = svn_atomic_read(&ns->min_used); i < count; ++i)
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if (strncmp(ns->data->atomics[i].name, name, len + 1) == 0)
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return_atomic(atomic, ns, i);
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* Serialize all lookup and insert the item, if necessary and allowed.
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SVN_ERR(lock(&ns->mutex));
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/* We only need to check for new entries.
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for (i = count; i < ns->data->count; ++i)
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if (strncmp(ns->data->atomics[i].name, name, len + 1) == 0)
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return_atomic(atomic, ns, i);
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/* Update our cached number of complete entries. */
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svn_atomic_set(&ns->min_used, ns->data->count);
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return unlock(&ns->mutex, error);
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/* Not found. Append a new entry, if allowed & possible.
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if (ns->data->count < MAX_ATOMIC_COUNT)
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ns->data->atomics[ns->data->count].value = 0;
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memcpy(ns->data->atomics[ns->data->count].name,
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return_atomic(atomic, ns, ns->data->count);
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error = svn_error_create(SVN_ERR_BAD_ATOMIC, 0,
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_("Out of slots for named atomic."));
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/* We are mainly done here. Let others continue their work.
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SVN_ERR(unlock(&ns->mutex, error));
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/* Only now can we be sure that a full memory barrier has been set
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* and that the new entry has been written to memory in full.
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svn_atomic_set(&ns->min_used, ns->data->count);
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svn_named_atomic__read(apr_int64_t *value,
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svn_named_atomic__t *atomic)
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SVN_ERR(validate(atomic));
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NA_SYNCHRONIZE(atomic, *value = synched_read(&atomic->data->value));
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svn_named_atomic__write(apr_int64_t *old_value,
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apr_int64_t new_value,
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svn_named_atomic__t *atomic)
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SVN_ERR(validate(atomic));
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NA_SYNCHRONIZE(atomic, temp = synched_write(&atomic->data->value, new_value));
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svn_named_atomic__add(apr_int64_t *new_value,
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svn_named_atomic__t *atomic)
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SVN_ERR(validate(atomic));
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NA_SYNCHRONIZE(atomic, temp = synched_add(&atomic->data->value, delta));
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svn_named_atomic__cmpxchg(apr_int64_t *old_value,
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apr_int64_t new_value,
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apr_int64_t comperand,
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svn_named_atomic__t *atomic)
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SVN_ERR(validate(atomic));
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NA_SYNCHRONIZE(atomic, temp = synched_cmpxchg(&atomic->data->value,
added
removed
subversion/libsvn_subr/named_atomic.c
