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/*
*
* Copyright 2015 gRPC authors.
*
* 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.
*
*/
#include "src/core/lib/support/stack_lockfree.h"
#include <stdlib.h>
#include <string.h>
#include <grpc/support/alloc.h>
#include <grpc/support/atm.h>
#include <grpc/support/log.h>
#include <grpc/support/port_platform.h>
/* The lockfree node structure is a single architecture-level
word that allows for an atomic CAS to set it up. */
struct lockfree_node_contents {
/* next thing to look at. Actual index for head, next index otherwise */
uint16_t index;
#ifdef GPR_ARCH_64
uint16_t pad;
uint32_t aba_ctr;
#else
#ifdef GPR_ARCH_32
uint16_t aba_ctr;
#else
#error Unsupported bit width architecture
#endif
#endif
};
/* Use a union to make sure that these are in the same bits as an atm word */
typedef union lockfree_node {
gpr_atm atm;
struct lockfree_node_contents contents;
} lockfree_node;
/* make sure that entries aligned to 8-bytes */
#define ENTRY_ALIGNMENT_BITS 3
/* reserve this entry as invalid */
#define INVALID_ENTRY_INDEX ((1 << 16) - 1)
struct gpr_stack_lockfree {
lockfree_node *entries;
lockfree_node head; /* An atomic entry describing curr head */
};
gpr_stack_lockfree *gpr_stack_lockfree_create(size_t entries) {
gpr_stack_lockfree *stack;
stack = (gpr_stack_lockfree *)gpr_malloc(sizeof(*stack));
/* Since we only allocate 16 bits to represent an entry number,
* make sure that we are within the desired range */
/* Reserve the highest entry number as a dummy */
GPR_ASSERT(entries < INVALID_ENTRY_INDEX);
stack->entries = (lockfree_node *)gpr_malloc_aligned(
entries * sizeof(stack->entries[0]), ENTRY_ALIGNMENT_BITS);
/* Clear out all entries */
memset(stack->entries, 0, entries * sizeof(stack->entries[0]));
memset(&stack->head, 0, sizeof(stack->head));
GPR_ASSERT(sizeof(stack->entries->atm) == sizeof(stack->entries->contents));
/* Point the head at reserved dummy entry */
stack->head.contents.index = INVALID_ENTRY_INDEX;
/* Fill in the pad and aba_ctr to avoid confusing memcheck tools */
#ifdef GPR_ARCH_64
stack->head.contents.pad = 0;
#endif
stack->head.contents.aba_ctr = 0;
return stack;
}
void gpr_stack_lockfree_destroy(gpr_stack_lockfree *stack) {
gpr_free_aligned(stack->entries);
gpr_free(stack);
}
int gpr_stack_lockfree_push(gpr_stack_lockfree *stack, int entry) {
lockfree_node head;
lockfree_node newhead;
lockfree_node curent;
lockfree_node newent;
/* First fill in the entry's index and aba ctr for new head */
newhead.contents.index = (uint16_t)entry;
#ifdef GPR_ARCH_64
/* Fill in the pad to avoid confusing memcheck tools */
newhead.contents.pad = 0;
#endif
/* Also post-increment the aba_ctr */
curent.atm = gpr_atm_no_barrier_load(&stack->entries[entry].atm);
newhead.contents.aba_ctr = ++curent.contents.aba_ctr;
gpr_atm_no_barrier_store(&stack->entries[entry].atm, curent.atm);
do {
/* Atomically get the existing head value for use */
head.atm = gpr_atm_no_barrier_load(&(stack->head.atm));
/* Point to it */
newent.atm = gpr_atm_no_barrier_load(&stack->entries[entry].atm);
newent.contents.index = head.contents.index;
gpr_atm_no_barrier_store(&stack->entries[entry].atm, newent.atm);
} while (!gpr_atm_rel_cas(&(stack->head.atm), head.atm, newhead.atm));
/* Use rel_cas above to make sure that entry index is set properly */
return head.contents.index == INVALID_ENTRY_INDEX;
}
int gpr_stack_lockfree_pop(gpr_stack_lockfree *stack) {
lockfree_node head;
lockfree_node newhead;
do {
head.atm = gpr_atm_acq_load(&(stack->head.atm));
if (head.contents.index == INVALID_ENTRY_INDEX) {
return -1;
}
newhead.atm =
gpr_atm_no_barrier_load(&(stack->entries[head.contents.index].atm));
} while (!gpr_atm_no_barrier_cas(&(stack->head.atm), head.atm, newhead.atm));
return head.contents.index;
}
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