/*
* Copyright (c) 2016 - 2017, NVIDIA CORPORATION. All rights reserved.
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
* DEALINGS IN THE SOFTWARE.
*/
/*
* Basics:
*
* - Lockless memory allocator for fixed-size structures, whose
* size is defined up front at init time.
* - Memory footprint scales linearly w/ the number of structures in
* the pool. It is ~= sizeof(int) * N.
* - Memory is pre-allocated by the client. The allocator itself
* only computes the addresses for allocations.
* - Limit of MAX_INT nodes that the allocator can be responsible for.
*
* Implementation details:
*
* The allocator maintains a single list of free nodes. We allocate &
* free nodes from the head of the list. We rely on the cmpxchg() operator
* to maintain atomicity on the head.
*
* So, both allocs & frees are O(1)!!
*
* -- Definitions --
* Block Size - size of a single structure that this allocator will
* allocate.
* Node - one of the elements of size blk_size in the
* client-allocated buffer.
* Node Index - zero-based index of a node in the client-allocated
* contiguous buffer.
*
* -- Initial State --
* We maintain the following to track the state of the free list:
*
* 1) A "head" index to track the index of the first free node in the list
* 2) A "next" array to track the index of the next free node in the list
* for every node. So next[head], will give the index to the 2nd free
* element in the list.
*
* So, to begin with, the free list consists of all node indices, and each
* position in the next array contains index N + 1:
*
* head = 0
* next = [1, 2, 3, 4, -1] : Example for a user-allocated buffer of 5 nodes
* free_list = 0->1->2->3->4->-1
*
* -- Allocations --
* 1) Read the current head (aka acq_head)
* 2) Read next[acq_head], to get the 2nd free element (aka new_head)
* 3) cmp_xchg(&head, acq_head, new_head)
* 4) If it succeeds, compute the address of the node, based on
* base address, blk_size, & acq_head.
*
* head = 1;
* next = [1, 2, 3, 4, -1] : Example after allocating Node #0
* free_list = 1->2->3->4->-1
*
* head = 2;
* next = [1, 2, 3, 4, -1] : Example after allocating Node #1
* free_list = 2->3->4->-1
*
* -- Frees --
* 1) Based on the address to be freed, calculate the index of the node
* being freed (cur_idx)
* 2) Read the current head (old_head)
* 3) So the freed node is going to go at the head of the list, and we
* want to put the old_head after it. So next[cur_idx] = old_head
* 4) cmpxchg(head, old_head, cur_idx)
*
* head = 0
* next = [2, 2, 3, 4, -1]
* free_list = 0->2->3->4->-1 : Example after freeing Node #0
*
* head = 1
* next = [2, 0, 3, 4, -1]
* free_list = 1->0->2->3->4->-1 : Example after freeing Node #1
*/
#ifndef LOCKLESS_ALLOCATOR_PRIV_H
#define LOCKLESS_ALLOCATOR_PRIV_H
struct nvgpu_allocator;
struct nvgpu_lockless_allocator {
struct nvgpu_allocator *owner;
u64 base; /* Base address of the space. */
u64 length; /* Length of the space. */
u64 blk_size; /* Size of the structure being allocated */
int nr_nodes; /* Number of nodes available for allocation */
int *next; /* An array holding the next indices per node */
int head; /* Current node at the top of the stack */
u64 flags;
bool inited;
/* Statistics */
nvgpu_atomic_t nr_allocs;
};
static inline struct nvgpu_lockless_allocator *lockless_allocator(
struct nvgpu_allocator *a)
{
return (struct nvgpu_lockless_allocator *)(a)->priv;
}
#endif
