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