From 34506b30e4210b417aa38d1518aa2c53fb7cf39b Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 15 Jul 2016 20:58:41 +0300 Subject: util/qht: Document memory ordering assumptions It is naturally expected that some memory ordering should be provided around qht_insert() and qht_lookup(). Document these assumptions in the header file and put some comments in the source to denote how that memory ordering requirements are fulfilled. Signed-off-by: Paolo Bonzini [Sergey Fedorov: commit title and message provided; comment on qht_remove() elided] Signed-off-by: Sergey Fedorov Message-Id: <20160715175852.30749-2-sergey.fedorov@linaro.org> Signed-off-by: Paolo Bonzini --- util/qht.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'util') diff --git a/util/qht.c b/util/qht.c index 40d6e218f7..28ce289245 100644 --- a/util/qht.c +++ b/util/qht.c @@ -445,7 +445,11 @@ void *qht_do_lookup(struct qht_bucket *head, qht_lookup_func_t func, do { for (i = 0; i < QHT_BUCKET_ENTRIES; i++) { if (b->hashes[i] == hash) { - void *p = atomic_read(&b->pointers[i]); + /* The pointer is dereferenced before seqlock_read_retry, + * so (unlike qht_insert__locked) we need to use + * atomic_rcu_read here. + */ + void *p = atomic_rcu_read(&b->pointers[i]); if (likely(p) && likely(func(p, userp))) { return p; @@ -535,6 +539,7 @@ static bool qht_insert__locked(struct qht *ht, struct qht_map *map, atomic_rcu_set(&prev->next, b); } b->hashes[i] = hash; + /* smp_wmb() implicit in seqlock_write_begin. */ atomic_set(&b->pointers[i], p); seqlock_write_end(&head->sequence); return true; -- cgit v1.2.3