71 results about "Read-copy-update" patented technology

A technique for realtime-safe detection of a grace period for deferring the destruction of a shared data element until pre-existing references to the data element have been removed. A grace period identifier is provided for readers of the shared data element to consult. A next grace period is initiated by manipulating the grace period identifier, and an acknowledgement thereof is requested from processing entities capable of executing the readers before detecting when a current grace period has ended. Optionally, when the end of the current grace period is determined, arrangement is made for a memory barrier shoot-down on processing entities capable of executing the readers. Data destruction operations to destroy the shared data element are then deferred until it is determined that the memory barriers have been implemented. Data destruction operations may be further deferred until two consecutive grace periods have expired.

A system, method and computer program product for atomically moving a shared list element from a first list location to a second list location includes inserting a placeholder element at the second list location to signify to readers that a move operation is underway, removing the shared list element from the first list location, re-identifying the list element to reflect its move from the first list location to the second list location, inserting it at the second list location and unlinking the placeholder element. A deferred removal of the placeholder element is performed following a period in which readers can no longer maintain references thereto. A method, system and computer program product are additionally provided for performing a lookup of a target list element that is subject to being atomically moved from a first list to a second list.

A technique for realtime-safe detection of a grace period for deferring the destruction of a shared data element until pre-existing references to the data element have been removed. A per-processor read / write lock is established for each of one or more processors. When reading a shared data element at a processor, the processor's read / write lock is acquired for reading, the shared data element is referenced, and the read / write lock that was acquired for reading is released. When starting a new grace period, all of the read / write locks are acquired for writing, a new grace period is started, and all of the read / write locks are released.

A method, system and computer program product for supporting concurrent updates to a shared data element group while preserving group integrity on behalf of one or more readers that are concurrently referencing group data elements without using locks or atomic instructions. Two or more updaters may be invoked to generate new group data elements. Each new data element created by the same updater is assigned a new generation number that is different than a global generation number associated with the data element group and which allows a reader of the data element group to determine whether the new data element is a correct version for the reader. The new generation numbers are different for each updater and assigned according to an order in which the updaters respectively begin update operations. The global generation number is updated so that when all of the updaters have completed data element update processing, the global generation number will correspond to the new generation number that is associated with the last of the updaters to begin update operations.

A system, method and computer program product for synchronizing updates to shared mutable data in a clustered data processing system. A data element update operation is performed at each node of the cluster while preserving a pre-update view of the shared mutable data, or an associated operational mode, on behalf of readers that may be utilizing the pre-update view. A request is made for detection of a grace period, and grace period detection processing is performed for detecting when the cluster-wide grace period has occurred. When it does, a deferred action associated with the update operation it taken, such as removal of a pre-update view of the data element or termination of an associated mode of operation.

A technique for efficiently boosting the priority of a preemptable data reader in order to eliminate impediments to grace period processing that defers the destruction of one or more shared data elements that may be referenced by the reader until the reader is no longer capable of referencing the data elements. Upon the reader being subject to preemption or blocking, it is determined whether the reader is in a read-side critical section referencing any of the shared data elements. If it is, the reader's priority is boosted in order to expedite completion of the critical section. The reader's priority is subsequently decreased after the critical section has completed. In this way, delays in grace period processing due to reader preemption within the critical section, which can result in an out-of-memory condition, can be minimized efficiently with minimal processing overhead.

A method for avoiding unnecessary grace period token processing while detecting a grace period without atomic instructions in a read-copy update subsystem or other processing environment that requires deferring removal of a shared data element until pre-existing references to the data element are removed. Detection of the grace period includes establishing a token to be circulated between processing entities sharing access to the data element. A grace period elapses whenever the token makes a round trip through the processing entities. A distributed indicator associated with each processing entity indicates whether there is a need to perform removal processing on any shared data element. The distributed indicator is processed at each processing entity before the latter engages in token processing. Token processing is performed only when warranted by the distributed indicator. In this way, unnecessary token processing can be avoided when the distributed indicator does not warrant such processing.

A method, system and computer program product for modifying data elements in a shared data element group that must be updated atomically for the benefit of readers requiring group integrity. A global generation number is associated with the data element group and each member receives a copy of this number when it is created. Each time an update is performed, the global generation number is incremented and the updated element's copy of this number is set to the same value. For each updated data element, a link is maintained from the new version to the pre-update version thereof, either directly or using pointer-forwarding entities. When a search is initiated, the current global generation number is referenced at the commencement of the search. As data elements in the group are traversed, the reader traverses the links between new and old data element versions to find a version having a matching generation number, if any. Following the occurrence of a grace period in which all readers have passed through quiescent states, all old data element versions are freed.

A system, method and computer program product for atomically moving a shared list element from a first list location to a second list location includes inserting a placeholder element at the second list location to signify to readers that a move operation is underway, removing the shared list element from the first list location, re-identifying the list element to reflect its move from the first list location to the second list location, inserting it at the second list location and unlinking the placeholder element. A deferred removal of the placeholder element is performed following a period in which readers can no longer maintain references thereto. A method, system and computer program product are additionally provided for performing a lookup of a target list element that is subject to being atomically moved from a first list to a second list.

A technique for low-power detection of a grace period for deferring the destruction of a shared data element until pre-existing references to the data element have been removed. A grace period processing action is implemented that requires a response from a processor that may be running a preemptible reader of said shared data element before further grace period processing can proceed. A power and reader status of the processor is also determined. Grace period processing may proceed despite the absence of a response from the processor if the power and reader status indicates that an actual response from the processor is unnecessary.

A technique for low-power detection of a grace period following a shared data element update operation that affects non-preemptible data readers. A grace period processing action is implemented that requires a processor that may be running a non-preemptible reader of the shared data element to pass through a quiescent state before further grace period processing can proceed. A power status of the processor is also determined. Further grace period processing may proceed without requiring the processor to pass through a quiescent state if the power status indicates that quiescent state processing by the processor is unnecessary.

A technique for implementing fast path grace period detection for deferring the destruction of a shared data element until pre-existing references to the data element are removed. A check is made, without using locks to exclude other updaters, for the presence of readers that are accessing the shared data elements. Grace period detection is terminated to initiate deferred destruction of the data element if there are no readers accessing the shared data element. If there are readers accessing the shared data element, a lock is implemented and another check is made for the presence of the readers.

A method, system and computer program product for modifying data elements in a shared data element group that must be updated atomically for the benefit of readers requiring group integrity. A global generation number is associated with the data element group and each member receives a copy of this number when it is created. Each time an update is performed, the global generation number is incremented and the updated element's copy of this number is set to the same value. For each updated data element, a link is maintained from the new version to the pre-update version thereof, either directly or using pointer-forwarding entities. When a search is initiated, the current global generation number is referenced at the commencement of the search. As data elements in the group are traversed, the reader traverses the links between new and old data element versions to find a version having a matching generation number, if any. Following the occurrence of a grace period in which all readers have passed through quiescent states, all old data element versions are freed.

A technique for optimizing grace period detection in a uniprocessor environment. An update operation is performed on a data element that is shared with non-preemptible readers of the data element. A call is issued to a synchronous grace period detection method. The synchronous grace period detection method performs synchronous grace period detection and returns from the call if the data processing system implements a multi-processor environment at the time of the call. The synchronous grace period detection determines the end of a grace period in which the readers have passed through a quiescent state and cannot be maintaining references to the pre-update view of the shared data. The synchronous grace period detection method returns from the call without performing grace period detection if the data processing system implements a uniprocessor environment at the time of the call.

A technique for determining if a processor in a multiprocessor system implementing a read-copy update (RCU) subsystem may be placed in low power state. The technique may include determining whether the processor has any RCU callbacks that are ready for invocation or the RCU subsystem requires grace period advancement processing from the processor. The processor may be placed in a low power state if either (1) a first condition holds wherein the processor has one or more pending RCU callbacks, but does not have any RCU callbacks that are ready for invocation and the RCU subsystem does not require grace period advancement processing from the processor, (2) a second condition holds wherein the processor does not have any pending RCU callbacks.

A method, system and computer program product for supporting concurrent updates to a shared data element group while preserving group integrity on behalf of one or more readers that are concurrently referencing group data elements without using locks or atomic instructions. Two or more updaters may be invoked to generate new group data elements. Each new data element created by the same updater is assigned a new generation number that is different than a global generation number associated with the data element group and which allows a reader of the data element group to determine whether the new data element is a correct version for the reader. The new generation numbers are different for each updater and assigned according to an order in which the updaters respectively begin update operations. The global generation number is updated so that when all of the updaters have completed data element update processing, the global generation number will correspond to the new generation number that is associated with the last of the updaters to begin update operations.

A technique for efficiently boosting the priority of a preemptable data reader while resolving races between the priority boosting and the reader exiting a critical section or terminating in order to eliminate impediments to grace period processing that defers the destruction of one or more shared data elements that may be referenced by the reader until the reader is no longer capable of referencing the one or more data elements. A determination is made that the reader is in a read-side critical section and the reader is designated as a candidate for priority boosting. A verification is made that the reader has not exited its critical section or terminated, and the reader's priority is boosted to expedite its completion of the critical section. The reader's priority is decreased following its completion of the critical section.

A technique for making a free-running grace period counter safe against lengthy low power state processor sojourns. The grace period counter tracks grace periods that determine when processors that are capable executing read operations have passed through a quiescent state that guarantees the readers will no longer maintain references to shared data. Periodically, one or more processors may be placed in a low power state in which the processors discontinue performing grace period detection operations. Such processors may remain in the low power state for a complete cycle of the grace period counter. This scenario can potentially disrupt grace period detection operations if the processors awaken to see the same grace period counter value. To rectify this situation, processors in a low power state may be periodically awakened at a predetermined point selected prevent the low power state from extending for an entire roll over of the grace period counter.

A technique for realtime-safe detection of a grace period for deferring the destruction of a shared data element until pre-existing references to the data element have been removed. A pair of counters is established for each of one or more processors. A global counter selector determines which counter of each per-processor counter pair is a current counter. When reading a shared data element at a processor, the processor's current counter is incremented. Following counter incrementation, the processor's counter pair is tested for reversal to ensure that the incremented counter is still the current counter. If a counter reversal has occurred, such that the incremented counter is no longer current, the processor's other counter is incremented. Following referencing of the shared data element, any counter that remains incremented is decremented. Following an update to the shared data element wherein a pre-update version of the element is maintained, the global counter selector is switched to establish a new current counter of each per-processor counter pair. The non-current counter of each per-processor counter pair is tested for zero. The shared data element's pre-update version is destroyed upon the non-current counter of each per-processor counter pair being zero.

A technique for realtime-safe detection of a grace period for deferring the destruction of a shared data element until pre-existing references to the data element have been removed. A per-processor read / write lock is established for each of one or more processors. When reading a shared data element at a processor, the processor's read / write lock is acquired for reading, the shared data element is referenced, and the read / write lock that was acquired for reading is released. When starting a new grace period, all of the read / write locks are acquired for writing, a new grace period is started, and all of the read / write locks are released.

A technique for managing read-copy update readers that have been preempted while executing in a read-copy update read-side critical section. A single blocked-tasks list is used to track preempted reader tasks that are blocking an asynchronous grace period, preempted reader tasks that are blocking an expedited grace period, and preempted reader tasks that require priority boosting. In example embodiments, a first pointer may be used to segregate the blocked-tasks list into preempted reader tasks that are and are not blocking a current asynchronous grace period. A second pointer may be used to segregate the blocked-tasks list into preempted reader tasks that are and are not blocking an expedited grace period. A third pointer may be used to segregate the blocked-tasks list into preempted reader tasks that do and do not require priority boosting.

A technique for expediting the unloading of an operating system kernel module that executes read-copy update (RCU) callback processing code in a computing system having one or more processors. According to embodiments of the disclosed technique, an RCU callback is enqueued so that it can be processed by the kernel module's callback processing code following completion of a grace period in which each of the one or more processors has passed through a quiescent state. An expediting operation is performed to expedite processing of the RCU callback. The RCU callback is then processed and the kernel module is unloaded.

A technique for implementing read-copy update in a shared-memory computing system having two or more processors operatively coupled to a shared memory and to associated incoherent caches that cache copies of data stored in the memory. According to example embodiments disclosed herein, cacheline information for data that has been rendered obsolete due to a data update being performed by one of the processors is recorded. The recorded cacheline information is communicated to one or more of the other processors. The one or more other processors use the communicated cacheline information to flush the obsolete data from all incoherent caches that may be caching such data.

A technique for expediting the unloading of an operating system kernel module that executes read-copy update (RCU) callback processing code in a computing system having one or more processors. According to embodiments of the disclosed technique, an RCU callback is enqueued so that it can be processed by the kernel module's callback processing code following completion of a grace period in which each of the one or more processors has passed through a quiescent state. An expediting operation is performed to expedite processing of the RCU callback. The RCU callback is then processed and the kernel module is unloaded.