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Switch to atomic for setting autocreated objects.

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- Update semaphore comment to new scope.
- Use an atomic swap to avoid needing to use the semaphore.

This means the semaphore is create only when extension are auto created (less
memory usage).
This commit is contained in:
Thomas Van Lenten 2020-10-22 15:58:46 -04:00
parent de5d1b98c2
commit 2123ed5df7
3 changed files with 76 additions and 60 deletions
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123
objectivec/GPBMessage.m
View File

@ -71,6 +71,8 @@ static NSString *const kGPBDataCoderKey = @"GPBData";
@package
GPBUnknownFieldSet *unknownFields_;
NSMutableDictionary *extensionMap_;
// Readonly access to autocreatedExtensionMap_ is protected via
// readOnlySemaphore_.
NSMutableDictionary *autocreatedExtensionMap_;
// If the object was autocreated, we remember the creator so that if we get
@ -79,10 +81,10 @@ static NSString *const kGPBDataCoderKey = @"GPBData";
GPBFieldDescriptor *autocreatorField_;
GPBExtensionDescriptor *autocreatorExtension_;
// A lock to provide mutual exclusion from internal data that can be modified
// by *read* operations such as getters (autocreation of message fields and
// message extensions, not setting of values). Used to guarantee thread safety
// for concurrent reads on the message.
// Message can only be mutated from one thread. But some *readonly* operations
// modifify internal state because they autocreate things. The
// autocreatedExtensionMap_ is one such structure. Access during readonly
// operations is protected via this semaphore.
// NOTE: OSSpinLock may seem like a good fit here but Apple engineers have
// pointed out that they are vulnerable to live locking on iOS in cases of
// priority inversion:
@ -583,19 +585,30 @@ static id GetOrCreateArrayIvarWithField(GPBMessage *self,
// This is like GPBGetObjectIvarWithField(), but for arrays, it should
// only be used to wire the method into the class.
static id GetArrayIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
id array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!array) {
// Check again after getting the lock.
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
array = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!array) {
array = CreateArrayForField(field, self);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, array);
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id array = atomic_load(typePtr);
if (array) {
return array;
}
return array;
id expected = nil;
id autocreated = CreateArrayForField(field, self);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
if (GPBFieldDataTypeIsObject(field)) {
GPBAutocreatedArray *autoArray = autocreated;
autoArray->_autocreator = nil;
} else {
GPBInt32Array *gpbArray = autocreated;
gpbArray->_autocreator = nil;
}
[autocreated release];
return expected;
}
static id GetOrCreateMapIvarWithField(GPBMessage *self,
@ -613,19 +626,31 @@ static id GetOrCreateMapIvarWithField(GPBMessage *self,
// This is like GPBGetObjectIvarWithField(), but for maps, it should
// only be used to wire the method into the class.
static id GetMapIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
id dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!dict) {
// Check again after getting the lock.
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
dict = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!dict) {
dict = CreateMapForField(field, self);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, dict);
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id dict = atomic_load(typePtr);
if (dict) {
return dict;
}
return dict;
id expected = nil;
id autocreated = CreateMapForField(field, self);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
if ((field.mapKeyDataType == GPBDataTypeString) &&
GPBFieldDataTypeIsObject(field)) {
GPBAutocreatedDictionary *autoDict = autocreated;
autoDict->_autocreator = nil;
} else {
GPBInt32Int32Dictionary *gpbDict = autocreated;
gpbDict->_autocreator = nil;
}
[autocreated release];
return expected;
}
#endif // !defined(__clang_analyzer__)
@ -3337,30 +3362,34 @@ id GPBGetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field) {
id GPBGetObjectIvarWithField(GPBMessage *self, GPBFieldDescriptor *field) {
NSCAssert(!GPBFieldIsMapOrArray(field), @"Shouldn't get here");
if (GPBGetHasIvarField(self, field)) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
return *typePtr;
}
// Not set...
// Non messages (string/data), get their default.
if (!GPBFieldDataTypeIsMessage(field)) {
if (GPBGetHasIvarField(self, field)) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
return *typePtr;
}
// Not set...non messages (string/data), get their default.
return field.defaultValue.valueMessage;
}
GPBPrepareReadOnlySemaphore(self);
dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
GPBMessage *result = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
if (!result) {
// For non repeated messages, create the object, set it and return it.
// This object will not initially be visible via GPBGetHasIvar, so
// we save its creator so it can become visible if it's mutated later.
result = GPBCreateMessageWithAutocreator(field.msgClass, self, field);
GPBSetAutocreatedRetainedObjectIvarWithField(self, field, result);
uint8_t *storage = (uint8_t *)self->messageStorage_;
_Atomic(id) *typePtr = (_Atomic(id) *)&storage[field->description_->offset];
id msg = atomic_load(typePtr);
if (msg) {
return msg;
}
dispatch_semaphore_signal(self->readOnlySemaphore_);
return result;
id expected = nil;
id autocreated = GPBCreateMessageWithAutocreator(field.msgClass, self, field);
if (atomic_compare_exchange_strong(typePtr, &expected, autocreated)) {
// Value was set, return it.
return autocreated;
}
// Some other thread set it, release the one created and return what got set.
GPBClearMessageAutocreator(autocreated);
[autocreated release];
return expected;
}
#pragma clang diagnostic pop

9
objectivec/GPBUtilities.m
View File

@ -504,15 +504,6 @@ static void GPBMaybeClearOneofPrivate(GPBMessage *self,
// Object types are handled slightly differently, they need to be released
// and retained.
void GPBSetAutocreatedRetainedObjectIvarWithField(
GPBMessage *self, GPBFieldDescriptor *field,
id __attribute__((ns_consumed)) value) {
uint8_t *storage = (uint8_t *)self->messageStorage_;
id *typePtr = (id *)&storage[field->description_->offset];
NSCAssert(*typePtr == NULL, @"Can't set autocreated object more than once.");
*typePtr = value;
}
void GPBClearAutocreatedMessageIvarWithField(GPBMessage *self,
GPBFieldDescriptor *field) {
if (GPBGetHasIvarField(self, field)) {

4
objectivec/GPBUtilities_PackagePrivate.h
View File

@ -289,10 +289,6 @@ void GPBSetRetainedObjectIvarWithFieldPrivate(GPBMessage *self,
id GPBGetObjectIvarWithFieldNoAutocreate(GPBMessage *self,
GPBFieldDescriptor *field);
void GPBSetAutocreatedRetainedObjectIvarWithField(
GPBMessage *self, GPBFieldDescriptor *field,
id __attribute__((ns_consumed)) value);
// Clears and releases the autocreated message ivar, if it's autocreated. If
// it's not set as autocreated, this method does nothing.
void GPBClearAutocreatedMessageIvarWithField(GPBMessage *self,