| 1 | // SPDX-FileCopyrightText: 2010 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> |
| 2 | // |
| 3 | // SPDX-License-Identifier: LGPL-2.1-or-later |
| 4 | |
| 5 | #ifndef _URCU_RCULFSTACK_STATIC_H |
| 6 | #define _URCU_RCULFSTACK_STATIC_H |
| 7 | |
| 8 | /* |
| 9 | * Userspace RCU library - Lock-Free RCU Stack |
| 10 | * |
| 11 | * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfstack.h for linking |
| 12 | * dynamically with the userspace rcu library. |
| 13 | */ |
| 14 | |
| 15 | #include <urcu/uatomic.h> |
| 16 | #include <urcu-pointer.h> |
| 17 | |
| 18 | #ifdef __cplusplus |
| 19 | extern "C" { |
| 20 | #endif |
| 21 | |
| 22 | static inline |
| 23 | void _cds_lfs_node_init_rcu(struct cds_lfs_node_rcu *node __attribute__((unused))) |
| 24 | { |
| 25 | } |
| 26 | |
| 27 | static inline |
| 28 | void _cds_lfs_init_rcu(struct cds_lfs_stack_rcu *s) |
| 29 | { |
| 30 | s->head = NULL; |
| 31 | } |
| 32 | |
| 33 | /* |
| 34 | * Lock-free stack push is not subject to ABA problem, so no need to |
| 35 | * take the RCU read-side lock. Even if "head" changes between two |
| 36 | * uatomic_cmpxchg() invocations here (being popped, and then pushed |
| 37 | * again by one or more concurrent threads), the second |
| 38 | * uatomic_cmpxchg() invocation only cares about pushing a new entry at |
| 39 | * the head of the stack, ensuring consistency by making sure the new |
| 40 | * node->next is the same pointer value as the value replaced as head. |
| 41 | * It does not care about the content of the actual next node, so it can |
| 42 | * very well be reallocated between the two uatomic_cmpxchg(). |
| 43 | * |
| 44 | * We take the approach of expecting the stack to be usually empty, so |
| 45 | * we first try an initial uatomic_cmpxchg() on a NULL old_head, and |
| 46 | * retry if the old head was non-NULL (the value read by the first |
| 47 | * uatomic_cmpxchg() is used as old head for the following loop). The |
| 48 | * upside of this scheme is to minimize the amount of cacheline traffic, |
| 49 | * always performing an exclusive cacheline access, rather than doing |
| 50 | * non-exclusive followed by exclusive cacheline access (which would be |
| 51 | * required if we first read the old head value). This design decision |
| 52 | * might be revisited after more thorough benchmarking on various |
| 53 | * platforms. |
| 54 | * |
| 55 | * Returns 0 if the stack was empty prior to adding the node. |
| 56 | * Returns non-zero otherwise. |
| 57 | */ |
| 58 | static inline |
| 59 | int _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s, |
| 60 | struct cds_lfs_node_rcu *node) |
| 61 | { |
| 62 | struct cds_lfs_node_rcu *head = NULL; |
| 63 | |
| 64 | for (;;) { |
| 65 | struct cds_lfs_node_rcu *old_head = head; |
| 66 | |
| 67 | node->next = head; |
| 68 | /* |
| 69 | * uatomic_cmpxchg() implicit memory barrier orders earlier |
| 70 | * stores to node before publication. |
| 71 | */ |
| 72 | cmm_emit_legacy_smp_mb(); |
| 73 | head = uatomic_cmpxchg_mo(&s->head, old_head, node, |
| 74 | CMM_SEQ_CST, CMM_SEQ_CST); |
| 75 | if (old_head == head) |
| 76 | break; |
| 77 | } |
| 78 | return (int) !!((unsigned long) head); |
| 79 | } |
| 80 | |
| 81 | /* |
| 82 | * Should be called under rcu read-side lock. |
| 83 | * |
| 84 | * The caller must wait for a grace period to pass before freeing the returned |
| 85 | * node or modifying the cds_lfs_node_rcu structure. |
| 86 | * Returns NULL if stack is empty. |
| 87 | */ |
| 88 | static inline |
| 89 | struct cds_lfs_node_rcu * |
| 90 | _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s) |
| 91 | { |
| 92 | for (;;) { |
| 93 | struct cds_lfs_node_rcu *head; |
| 94 | |
| 95 | head = rcu_dereference(s->head); |
| 96 | if (head) { |
| 97 | struct cds_lfs_node_rcu *next = rcu_dereference(head->next); |
| 98 | |
| 99 | if (uatomic_cmpxchg_mo(&s->head, head, next, |
| 100 | CMM_SEQ_CST, CMM_SEQ_CST) == head) { |
| 101 | cmm_emit_legacy_smp_mb(); |
| 102 | return head; |
| 103 | } else { |
| 104 | /* Concurrent modification. Retry. */ |
| 105 | continue; |
| 106 | } |
| 107 | } else { |
| 108 | /* Empty stack */ |
| 109 | return NULL; |
| 110 | } |
| 111 | } |
| 112 | } |
| 113 | |
| 114 | #ifdef __cplusplus |
| 115 | } |
| 116 | #endif |
| 117 | |
| 118 | #endif /* _URCU_RCULFSTACK_STATIC_H */ |