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/* IBM_PROLOG_BEGIN_TAG */
/* This is an automatically generated prolog. */
/* */
/* $Source: src/include/util/lockfree/abaptr.H $ */
/* */
/* OpenPOWER HostBoot Project */
/* */
/* Contributors Listed Below - COPYRIGHT 2015,2017 */
/* [+] International Business Machines Corp. */
/* */
/* */
/* Licensed 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. */
/* */
/* IBM_PROLOG_END_TAG */
#ifndef __UTIL_LOCKFREE_ABAPTR_H
#define __UTIL_LOCKFREE_ABAPTR_H
// This code makes assumptions on pointer sizes that are not valid within
// our runtime environment, therefore it cannot ever be used there.
#ifdef __HOSTBOOT_RUNTIME
#error "AbaPtr is not allowed in HBRT"
#endif
#include <stdint.h>
#include <assert.h>
namespace Util
{
namespace Lockfree
{
/** ABA-safe smart pointer class.
*
* Lockfree algorithms often utilize atomic operations like
* 'compare-and-swap' to identify conditions when another thread
* has caused contention and already modified the same data this
* thread is attempting to modify. The 'ABA' problem, or 'A-B-A',
* is a common problem class in lockfree algorithms.
*
* The desired behavior between two threads is a sequence such as:
* 1) 'A' reads a value and begins some calculation.
* 2) 'B' updates the value read by 'A'.
* 3) 'A' attempts to atomically update the value but fails
* because 'B' has already modified the value in step 2.
*
* The typical solution to this program sequence is an atomic update,
* like compare-and-swap, that fails when the value is different
* between steps 1 and 3.
*
* When operating with pointers, it is quite possible for a pointer
* to be reused. Due to pointer reuse, the step 2 update could
* write the same value back as what was read in step 1. If this
* happens, step 3 will appear to succeed even though it should not.
* This is the ABA problem. (ABA affects any atomic algorithm where
* values can be reused, but especially impacts pointers)
*
* A common 'trick' to solve the ABA problem with pointers, and the
* one implemented here, is to use a rolling value in the upper
* part of a pointer. The upper N bits are reserved for this rolling
* value and the bottom 64-N bits can be masked off to obtain the
* real pointer value. N-bits of rolling value results in a
* (1 / 2^N) probability of an incorrectly-successful step 3 when
* there is pointer reuse.
*
* Since Hostboot heap memory addresses are all well below 4GB, we
* can use the upper 32 bits as the rolling value. This yields us
* probability sufficiently close to zero of a pointer reuse at the
* same time as a value reuse.
*
* This class acts as a smart-pointer specifically for maintaining
* the ABA-solution properties.
*/
template <typename _T>
class AbaPtr
{
public:
/** Default constructor - set everything to NULL. */
AbaPtr() : original(NULL), ptr(NULL), counter(0) {};
/** Construct from existing pointer value. */
explicit AbaPtr(_T* i_ptr) : original(i_ptr),
ptr(_getPtr(i_ptr)),
counter(_getCounter(i_ptr))
{
}
/** Assignment operator */
AbaPtr& operator=(_T* i_ptr)
{
original = i_ptr;
ptr = _getPtr(i_ptr);
counter = _getCounter(i_ptr);
return *this;
}
/** Set pointer portion. Update count. */
AbaPtr& set(_T* i_ptr)
{
ptr = _getPtr(i_ptr);
counter++;
return *this;
}
/** Get pointer. */
_T* get() const
{
return ptr;
}
/** Dereference operator. */
_T& operator*() const
{
return *ptr;
}
/** Dereference operator. */
_T* operator->() const
{
return ptr;
}
/** Perform atomic update of pointer, with ABA properties.
*
* @param[in] o_ptr - Pointer location to be updated.
*
* Updates o_ptr with the ABA pointer value from this
* object if the o_ptr still contains the 'original'
* value from time of construction or assignment.
*/
bool update(_T** o_ptr)
{
return __sync_bool_compare_and_swap(o_ptr,
original,
_getVal());
}
bool update(_T* volatile* o_ptr)
{
return __sync_bool_compare_and_swap(o_ptr,
original,
_getVal());
}
/** Get count */
uint32_t count() const
{
return counter;
}
/** Set count */
void setCount(uint32_t i_count)
{
counter = i_count;
}
/** Get full value */
_T* value() const { return _getVal(); }
protected:
/** Utility function to get 'pointer' part of ABA value. */
static _T* _getPtr(_T* i_ptr)
{
CPPASSERT(sizeof(_T*) == sizeof(uint64_t));
return reinterpret_cast<_T*>(
reinterpret_cast<uint64_t>(i_ptr) & 0x0FFFFFFFFull
);
}
/** Utility function to get 'counter' part of ABA value. */
static uint32_t _getCounter(_T* i_ptr)
{
CPPASSERT(sizeof(_T*) == sizeof(uint64_t));
return (reinterpret_cast<uint64_t>(i_ptr) >> 32) &
0x0FFFFFFFFull;
}
/** Reconstruct ABA poitner value from components. */
_T* _getVal() const
{
CPPASSERT(sizeof(_T*) == sizeof(uint64_t));
return reinterpret_cast<_T*>(
(static_cast<uint64_t>(counter) << 32) |
(reinterpret_cast<uint64_t>(ptr))
);
}
private:
/** Prevent operator== because it isn't intuitive for this
* object. */
bool operator==(const AbaPtr<_T>& i) const;
_T* original;
_T* ptr;
uint32_t counter;
};
}
}
#endif
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