From: Mathieu Desnoyers Date: Thu, 18 Aug 2011 16:13:44 +0000 (-0400) Subject: rculfstack: document "push" X-Git-Tag: v0.6.5~45 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=79dba87985c7dda26ae56e4c30451acea6b00bdd;p=userspace-rcu.git rculfstack: document "push" Signed-off-by: Mathieu Desnoyers --- diff --git a/urcu/static/rculfstack.h b/urcu/static/rculfstack.h index ed6b0d4..9d69fa7 100644 --- a/urcu/static/rculfstack.h +++ b/urcu/static/rculfstack.h @@ -43,6 +43,28 @@ void _cds_lfs_init_rcu(struct cds_lfs_stack_rcu *s) s->head = NULL; } +/* + * Lock-free stack push is not subject to ABA problem, so no need to + * take the RCU read-side lock. Even if "head" changes between two + * uatomic_cmpxchg() invocations here (being popped, and then pushed + * again by one or more concurrent threads), the second + * uatomic_cmpxchg() invocation only cares about pushing a new entry at + * the head of the stack, ensuring consistency by making sure the new + * node->next is the same pointer value as the value replaced as head. + * It does not care about the content of the actual next node, so it can + * very well be reallocated between the two uatomic_cmpxchg(). + * + * We take the approach of expecting the stack to be usually empty, so + * we first try an initial uatomic_cmpxchg() on a NULL old_head, and + * retry if the old head was non-NULL (the value read by the first + * uatomic_cmpxchg() is used as old head for the following loop). The + * upside of this scheme is to minimize the amount of cacheline traffic, + * always performing an exclusive cacheline access, rather than doing + * non-exclusive followed by exclusive cacheline access (which would be + * required if we first read the old head value). This design decision + * might be revisited after more throrough benchmarking on various + * platforms. + */ static inline void _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s, struct cds_lfs_node_rcu *node) {