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util/qht: Document memory ordering assumptions

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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 <pbonzini@redhat.com>
[Sergey Fedorov: commit title and message provided;
comment on qht_remove() elided]
Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-Id: <20160715175852.30749-2-sergey.fedorov@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2016-07-15 20:58:41 +03:00
parent cc0100f464
commit 34506b30e4
2 changed files with 11 additions and 1 deletions
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5
include/qemu/qht.h
View File

@ -69,6 +69,9 @@ void qht_destroy(struct qht *ht);
* Attempting to insert a NULL @p is a bug.
* Inserting the same pointer @p with different @hash values is a bug.
*
* In case of successful operation, smp_wmb() is implied before the pointer is
* inserted into the hash table.
*
* Returns true on sucess.
* Returns false if the @p-@hash pair already exists in the hash table.
*/
@ -83,6 +86,8 @@ bool qht_insert(struct qht *ht, void *p, uint32_t hash);
*
* Needs to be called under an RCU read-critical section.
*
* smp_read_barrier_depends() is implied before the call to @func.
*
* The user-provided @func compares pointers in QHT against @userp.
* If the function returns true, a match has been found.
*

7
util/qht.c
View File

@ -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;