Abstract: 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.

Abstract: A technique for enabling hardware transactional memory (HTM) to work more efficiently with readers that can tolerate stale data. In an embodiment, a pre-transaction load request is received from one of the readers, the pre-transaction load request signifying that the reader can tolerate pre-transaction data. A determination is made whether the pre-transaction load request comprises data that has been designated for update by a concurrent HTM transaction. If so, a cache line containing the data is marked as pre-transaction data. The concurrent HTM transaction proceeds without aborting notwithstanding the pre-transaction load request.

Abstract: A technique for supporting user mode specification of RCU grace period latency to an operating system kernel-level RCU implementation. Non-expedited and expedited RCU grace period mechanisms are provided for invocation by RCU updaters performing RCU update operations to respectively initiate non-expedited and expedited grace periods. An expedited grace period indicator in a kernel memory space is provided for indicating whether a non-expedited RCU grace period or an expedited RCU grace period should be invoked. The non-expedited RCU grace period mechanism is adapted to check the expedited grace period indicator, and if an expedited RCU grace period is indicated, to invoke the expedited grace period mechanism. A communication mechanism is provided for use by a user mode application executing in a user memory space to manipulate the expedited grace period indicator in the kernel memory space, and thereby control whether an expedited or non-expedited RCU grace period should be used.

Abstract: A technique for enabling hardware transactional memory (HTM) to work more efficiently with readers that can tolerate stale data. In an embodiment, a pre-transaction load request is received from one of the readers, the pre-transaction load request signifying that the reader can tolerate pre-transaction data. A determination is made whether the pre-transaction load request comprises data that has been designated for update by a concurrent HTM transaction. If so, a cache line containing the data is marked as pre-transaction data. The concurrent HTM transaction proceeds without aborting notwithstanding the pre-transaction load request.

Abstract: A technique for implementing SRCU with reduced OS jitter may include: (1) providing a pair of critical section counters for each CPU; (2) when entering an SRCU read-side critical section, incrementing one of the critical section counters associated with a first grace period; (3) when exiting an SRCU read-side critical section, decrementing one of the critical section counters associated with the first grace period; (4) when performing a data update, initiating the second grace period and performing a counter summation operation that sums the critical section counters associated with the first grace period to generate a critical section counter sum; (5) storing a snapshot value for each critical section counter during the summing; and (6) if the critical section counter sum indicates there are no active SRCU read-side critical sections for the first grace period, rechecking by comparing the snapshot values to current values of the critical section counters.

Abstract: A technique for motivating lazy RCU callbacks under out-of-memory conditions. In response to detecting an actual or potential OOM condition, non-lazy callback processing is performed for all processors whose RCU callback lists are non-empty due to at least one callback permitting lazy callback processing being present.

Abstract: A system, method and computer program product for resizing a hash table while supporting hash table scalability and concurrency. The hash table has one or more hash buckets each containing one or more items that are chained together in a linked list. Each item in the hash table is processed to determine if the item requires relocation from a first bucket associated with a first table size to second bucket associated with a second table size. If the item requires relocation, it is linked to the second bucket without moving or copying the item in memory. The item is unlinked from the first bucket after waiting until there is no current hash table reader whose search of the hash table could be affected by the unlinking, again without moving or copying the item in memory.

Abstract: 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.

Abstract: A technique for improving the performance of RCU-based searches and updates to a shared data element group where readers must see consistent data with respect to the group as a whole. An updater creates one or more new group data elements and assigns each element a new generation number that is different than a global generation number associated with the data element group, allowing readers to track update versions. The updater links the new data elements into the data element group and then updates the global generation number so that referential integrity is maintained. This is done using a generation number element that is referenced by a header pointer for the data element group, and which in turn references or forms part of one of the data elements. After a grace period has elapsed, the any prior version of the generation number element may be freed.

Abstract: A technique for improving the performance of RCU-based searches and updates to a shared data element group where readers must see consistent data with respect to the group as a whole. An updater creates one or more new group data elements and assigns each element a new generation number that is different than a global generation number associated with the data element group, allowing readers to track update versions. The updater links the new data elements into the data element group and then updates the global generation number so that referential integrity is maintained. This is done using a generation number element that is referenced by a header pointer for the data element group, and which in turn references or forms part of one of the data elements. After a grace period has elapsed, the any prior version of the generation number element may be freed.

Abstract: A technique for resolving deadlocks between an RCU subsystem and an operating system scheduler. An RCU reader manipulates a counter when entering and exiting an RCU read-side critical section. At the entry, the counter is incremented. At the exit, the counter is manipulated differently depending on the counter value. A first counter manipulation path is taken when the counter indicates a task-context RCU reader is exiting an outermost RCU read-side critical section. This path includes condition-based processing that may result in invocation of the operating system scheduler. The first path further includes a deadlock protection operation that manipulates the counter to prevent an intervening RCU reader from taking the same path. The second manipulation path is taken when the counter value indicates a task-context RCU reader is exiting a non-outermost RCU read-side critical section, or an RCU reader is nested within the first path. This path bypasses the condition-based processing.

Abstract: 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.

Abstract: Data writers desiring to update data without unduly impacting concurrent readers perform a synchronization operation with respect to plural processors or execution threads. The synchronization operation is parallelized using a hierarchical tree having a root node, one or more levels of internal nodes and as many leaf nodes as there are processors or threads. The tree is traversed from the root node to a lowest level of the internal nodes and the following node processing is performed for each node: (1) check the node's children, (2) if the children are leaf nodes, perform the synchronization operation relative to each leaf node's associated processor or thread, and (3) if the children are internal nodes, fan out and repeat the node processing with each internal node representing a new root node. The foregoing node processing is continued until all processors or threads associated with the leaf nodes have performed the synchronization operation.

Abstract: Concurrent resizing and modification of a first RCU-protected hash table includes allocating a second RCU-protected hash table, populating it by linking each hash bucket of the second hash table to all hash buckets of the first hash table containing elements that hash to the second hash table bucket, and publishing the second hash table. If the modifying comprises insertion, a new element is inserted at the head of a corresponding bucket in the second hash table. If the modifying comprises deletion, then within an RCU read-side critical section: (1) all pointers in hash buckets of the first and second hash tables that reference the element being deleted are removed or redirected, and (2) the element is freed following a grace period that protects reader references to the deleted element. The first table is freed from memory after awaiting a grace period that protects reader references to the first hash table.

Abstract: Concurrent resizing and modification of a first RCU-protected hash table includes allocating a second RCU-protected hash table, populating it by linking each hash bucket of the second hash table to all hash buckets of the first hash table containing elements that hash to the second hash table bucket, and publishing the second hash table. If the modifying comprises insertion, a new element is inserted at the head of a corresponding bucket in the second hash table. If the modifying comprises deletion, then within an RCU read-side critical section: (1) all pointers in hash buckets of the first and second hash tables that reference the element being deleted are removed or redirected, and (2) the element is freed following a grace period that protects reader references to the deleted element. The first table is freed from memory after awaiting a grace period that protects reader references to the first hash table.

Abstract: 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.

Abstract: A technique for determining if a processor in a multiprocessor system implementing a read-copy update (RCU) subsystem may be placed in a low power state. The technique may include performing a first predictive query of the RCU subsystem to request permission for the processor to enter the low power state. If permission is denied, the processor is not placed in the low power state. If permission is granted, the processor is placed in the low power state for a non-fixed duration. Regardless whether permission is denied or granted, a second confirming query of the RCU subsystem is performed to redetermined whether it is permissible for the processor to be in the low power state.

Abstract: 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.

Abstract: A technique for resolving deadlocks between an RCU subsystem and an operating system scheduler. An RCU reader manipulates a counter when entering and exiting an RCU read-side critical section. At the entry, the counter is incremented. At the exit, the counter is manipulated differently depending on the counter value. A first counter manipulation path is taken when the counter indicates a task-context RCU reader is exiting an outermost RCU read-side critical section. This path includes condition-based processing that may result in invocation of the operating system scheduler. The first path further includes a deadlock protection operation that manipulates the counter to prevent an intervening RCU reader from taking the same path. The second manipulation path is taken when the counter value indicates a task-context RCU reader is exiting a non-outermost RCU read-side critical section, or an RCU reader is nested within the first path. This path bypasses the condition-based processing.

Abstract: 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.