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Support for off-loop read-ony qp-trie transactions

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It is sometimes necessary to access a qp-trie outside an isc_loop,
such as in tests or an isc_work callback. The best option was to use
a `dns_qpmulti_write()` transaction, but that has overheads that are
not necessary for read-only access, such as committing a new version
of the trie even when nothing changed.

So this commit adds a `dns_qpmulti_read()` transaction, which is
nearly as lightweight as a query transaction, but it takes the mutex
like a write transaction.
This commit is contained in:
Tony Finch
2023-03-02 13:30:24 +00:00
parent fa1b57ee6e
commit 44c80c4ae1
2 changed files with 50 additions and 5 deletions
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25
lib/dns/include/dns/qp.h
View File

@@ -53,6 +53,12 @@
* lifetime of a `dns_qpread_t`, instead of using locks. Readers are
* not blocked by any write activity, and vice versa.
*
* For read-only access outside the scope of a loop, such as from an
* isc_work callback, `dns_qpmulti_lockedread()`. This looks like a
* query transaction; the difference is that a locked read transaction
* takes the `dns_qpmulti_t` mutex. When you have finished with a
* `dns_qpread_t`, call `dns_qpread_destroy()` to release the mutex.
*
* For reads that need a stable view of the trie for multiple cycles
* of an isc_loop, or which can be used from any thread, call
* `dns_qpmulti_snapshot()` to get a `dns_qpsnap_t`. A snapshot is for
@@ -598,10 +604,27 @@ dns_qpmulti_query(dns_qpmulti_t *multi, dns_qpread_t *qpr);
* \li `qpr` is a valid read-only qp-trie handle
*/
void
dns_qpmulti_lockedread(dns_qpmulti_t *multi, dns_qpread_t *qpr);
/*%<
* Start a read-only transaction that takes the `dns_qpmulti_t` mutex.
*
* The `dns_qpmulti_lockedread()` function must NOT be called from an
* isc_loop thread. We keep query and read transactions separate to
* avoid accidentally taking or failing to take the mutex.
*
* Requires:
* \li `multi` is a pointer to a valid multi-threaded qp-trie
* \li `qpr != NULL`
*
* Returns:
* \li `qpr` is a valid read-only qp-trie handle
*/
void
dns_qpread_destroy(dns_qpmulti_t *multi, dns_qpread_t *qpr);
/*%<
* End a lightweight read transaction.
* End a lightweight query or read transaction.
*
* Requires:
* \li `multi` is a pointer to a valid multi-threaded qp-trie

30
lib/dns/qp.c
View File

@@ -1282,12 +1282,31 @@ dns_qpmulti_query(dns_qpmulti_t *multi, dns_qpread_t *qp) {
REQUIRE(QPMULTI_VALID(multi));
REQUIRE(qp != NULL);
dns_qpmulti_t *whence = reader_open(multi, qp);
INSIST(whence == multi);
/* we must be in an isc_loop thread */
/* we MUST be in an isc_loop thread */
qp->tid = isc_tid();
REQUIRE(qp->tid != ISC_TID_UNKNOWN);
dns_qpmulti_t *whence = reader_open(multi, qp);
INSIST(whence == multi);
}
/*
* a locked read takes the mutex
*/
void
dns_qpmulti_lockedread(dns_qpmulti_t *multi, dns_qpread_t *qp) {
REQUIRE(QPMULTI_VALID(multi));
REQUIRE(qp != NULL);
/* we MUST NOT be in an isc_loop thread */
qp->tid = isc_tid();
REQUIRE(qp->tid == ISC_TID_UNKNOWN);
LOCK(&multi->mutex);
dns_qpmulti_t *whence = reader_open(multi, qp);
INSIST(whence == multi);
}
void
@@ -1295,6 +1314,9 @@ dns_qpread_destroy(dns_qpmulti_t *multi, dns_qpread_t *qp) {
REQUIRE(QPMULTI_VALID(multi));
REQUIRE(QP_VALID(qp));
REQUIRE(qp->tid == isc_tid());
if (qp->tid == ISC_TID_UNKNOWN) {
UNLOCK(&multi->mutex);
}
*qp = (dns_qpread_t){};
}