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Similar to the algorithm presented in https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf Not completely lock-free at the moment. Also the problems with epoch based memory reclamation are still present - a thread actively observing an epoch getting stuck will prevent LloadConnections and LloadOperations being freed, potentially running out of memory.
144 lines
4.6 KiB
C
144 lines
4.6 KiB
C
/* epoch.h - epoch based memory reclamation */
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/* $OpenLDAP$ */
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/* This work is part of OpenLDAP Software <http://www.openldap.org/>.
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*
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* Copyright 2018-2020 The OpenLDAP Foundation.
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted only as authorized by the OpenLDAP
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* Public License.
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*
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* A copy of this license is available in the file LICENSE in the
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* top-level directory of the distribution or, alternatively, at
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* <http://www.OpenLDAP.org/license.html>.
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*/
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#ifndef __LLOAD_EPOCH_H
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#define __LLOAD_EPOCH_H
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/** @file epoch.h
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*
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* Implementation of epoch based memory reclamation, in principle
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* similar to the algorithm presented in
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* https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf
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*/
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typedef uintptr_t epoch_t;
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/** @brief A callback function used to free object and associated data */
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typedef void (dispose_cb)( void *object );
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/** @brief Initiate global state */
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void epoch_init( void );
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/** @brief Finalise global state and free any objects still pending */
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void epoch_shutdown( void );
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/** @brief Register thread as active
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*
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* In order to safely access managed objects, a thread should call
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* this function or make sure no other thread is running (e.g. config
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* pause, late shutdown). After calling this, it is guaranteed that no
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* reachable objects will be freed before all threads have called
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* `epoch_leave( current_epoch + 1 )` so it is essential that there
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* is an upper limit to the amount of time between #epoch_join and
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* corresponding #epoch_leave or the number of unfreed objects might
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* grow without bounds.
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*
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* To simplify locking, memory is only freed when the current epoch
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* is advanced rather than on leaving it.
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*
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* Can be safely called multiple times by the same thread as long as
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* a matching #epoch_leave() call is made eventually.
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*
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* @return The observed epoch, to be passed to #epoch_leave()
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*/
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epoch_t epoch_join( void );
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/** @brief Register thread as inactive
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*
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* A thread should call this after they are finished with work
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* performed since matching call to #epoch_join(). It is not safe
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* to keep a local reference to managed objects after this call
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* unless other precautions have been made to prevent it being
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* released.
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*
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* @param[in] epoch Epoch identifier returned by a previous call to
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* #epoch_join().
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*/
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void epoch_leave( epoch_t epoch );
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/** @brief Return an unreachable object to be freed
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*
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* The object should already be unreachable at the point of call and
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* cb will be invoked when no other thread that could have seen it
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* is active any more. This happens when we have advanced by two
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* epochs.
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*
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* @param[in] ptr Object to be released/freed
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* @param[in] cb Callback to invoke when safe to do so
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*/
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void epoch_append( void *ptr, dispose_cb *cb );
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/**
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* \defgroup Reference counting helpers
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*/
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/**@{*/
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/** @brief Acquire a reference if possible
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*
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* Atomically, check reference count is non-zero and increment if so.
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* Returns old reference count.
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*
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* @param[in] refp Pointer to a reference counter
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* @return 0 if reference was already zero, non-zero if reference
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* count was successfully incremented
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*/
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int acquire_ref( uintptr_t *refp );
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/** @brief Check reference count and try to decrement
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*
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* Atomically, decrement reference count if non-zero and register
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* object if decremented to zero. Returning previous reference count.
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*
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* @param[in] refp Pointer to a reference counter
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* @param[in] object The managed object
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* @param[in] cb Callback to invoke when safe to do so
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* @return 0 if reference was already zero, non-zero if reference
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* count was non-zero at the time of call
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*/
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int try_release_ref( uintptr_t *refp, void *object, dispose_cb *cb );
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/** @brief Read reference count
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*
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* @param[in] object Pointer to the managed object
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* @param[in] ref_field Member where reference count is stored in
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* the object
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* @return Current value of reference counter
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*/
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#define IS_ALIVE( object, ref_field ) \
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__atomic_load_n( &(object)->ref_field, __ATOMIC_ACQUIRE )
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/** @brief Release reference
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*
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* A cheaper alternative to #try_release_ref(), safe only when we know
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* reference count was already non-zero.
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*
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* @param[in] object The managed object
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* @param[in] ref_field Member where reference count is stored in
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* the object
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* @param[in] cb Callback to invoke when safe to do so
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*/
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#define RELEASE_REF( object, ref_field, cb ) \
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do { \
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if ( !__atomic_sub_fetch( \
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&(object)->ref_field, 1, __ATOMIC_ACQ_REL ) ) { \
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epoch_append( object, (dispose_cb *)cb ); \
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} \
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} while (0)
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/**@}*/
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#endif /* __LLOAD_EPOCH_H */
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