be able to detect. Buffer context locks are used for short-term access
control to individual pages of the index.
-We define the following lmgr locks for a hash index:
-
-LockPage(rel, 0) represents the right to modify the hash-code-to-bucket
-mapping. A process attempting to enlarge the hash table by splitting a
-bucket must exclusive-lock this lock before modifying the metapage data
-representing the mapping. Processes intending to access a particular
-bucket must share-lock this lock until they have acquired lock on the
-correct target bucket.
-
LockPage(rel, page), where page is the page number of a hash bucket page,
represents the right to split or compact an individual bucket. A process
splitting a bucket must exclusive-lock both old and new halves of the
(It is okay to allow concurrent scans and insertions.)
The lmgr lock IDs corresponding to overflow pages are currently unused.
-These are available for possible future refinements.
+These are available for possible future refinements. LockPage(rel, 0)
+is also currently undefined (it was previously used to represent the right
+to modify the hash-code-to-bucket mapping, but it is no longer needed for
+that purpose).
Note that these lock definitions are conceptually distinct from any sort
of lock on the pages whose numbers they share. A process must also obtain
waiting for an unrelated lock held by another process. If that process
then does something that requires exclusive lock on the bucket, we have
deadlock. Therefore the bucket locks must be lmgr locks so that deadlock
-can be detected and recovered from. This also forces the page-zero lock
-to be an lmgr lock, because as we'll see below it is held while attempting
-to acquire a bucket lock, and so it could also participate in a deadlock.
+can be detected and recovered from.
Processes must obtain read (share) buffer context lock on any hash index
page while reading it, and write (exclusive) lock while modifying it.
The reader algorithm is:
- share-lock page 0 (to prevent active split)
- read/sharelock meta page
- compute bucket number for target hash key
- release meta page
- share-lock bucket page (to prevent split/compact of this bucket)
- release page 0 share-lock
+ pin meta page and take buffer content lock in shared mode
+ loop:
+ compute bucket number for target hash key
+ release meta page buffer content lock
+ if (correct bucket page is already locked)
+ break
+ release any existing bucket page lock (if a concurrent split happened)
+ take heavyweight bucket lock
+ retake meta page buffer content lock in shared mode
-- then, per read request:
- read/sharelock current page of bucket
+ release pin on metapage
+ read current page of bucket and take shared buffer content lock
step to next page if necessary (no chaining of locks)
get tuple
- release current page
+ release buffer content lock and pin on current page
-- at scan shutdown:
release bucket share-lock
-By holding the page-zero lock until lock on the target bucket is obtained,
-the reader ensures that the target bucket calculation is valid (otherwise
-the bucket might be split before the reader arrives at it, and the target
-entries might go into the new bucket). Holding the bucket sharelock for
+We can't hold the metapage lock while acquiring a lock on the target bucket,
+because that might result in an undetected deadlock (lwlocks do not participate
+in deadlock detection). Instead, we relock the metapage after acquiring the
+bucket page lock and check whether the bucket has been split. If not, we're
+done. If so, we release our previously-acquired lock and repeat the process
+using the new bucket number. Holding the bucket sharelock for
the remainder of the scan prevents the reader's current-tuple pointer from
being invalidated by splits or compactions. Notice that the reader's lock
does not prevent other buckets from being split or compacted.
The insertion algorithm is rather similar:
- share-lock page 0 (to prevent active split)
- read/sharelock meta page
- compute bucket number for target hash key
- release meta page
- share-lock bucket page (to prevent split/compact of this bucket)
- release page 0 share-lock
+ pin meta page and take buffer content lock in shared mode
+ loop:
+ compute bucket number for target hash key
+ release meta page buffer content lock
+ if (correct bucket page is already locked)
+ break
+ release any existing bucket page lock (if a concurrent split happened)
+ take heavyweight bucket lock in shared mode
+ retake meta page buffer content lock in shared mode
-- (so far same as reader)
- read/exclusive-lock current page of bucket
+ release pin on metapage
+ pin current page of bucket and take exclusive buffer content lock
if full, release, read/exclusive-lock next page; repeat as needed
>> see below if no space in any page of bucket
insert tuple at appropriate place in page
- write/release current page
- release bucket share-lock
- read/exclusive-lock meta page
+ mark current page dirty and release buffer content lock and pin
+ release heavyweight share-lock
+ pin meta page and take buffer content lock in shared mode
increment tuple count, decide if split needed
- write/release meta page
+ mark meta page dirty and release buffer content lock and pin
done if no split needed, else enter Split algorithm below
To speed searches, the index entries within any individual index page are
The algorithm attempts, but does not necessarily succeed, to split one
existing bucket in two, thereby lowering the fill ratio:
- exclusive-lock page 0 (assert the right to begin a split)
- read/exclusive-lock meta page
+ pin meta page and take buffer content lock in exclusive mode
check split still needed
- if split not needed anymore, drop locks and exit
+ if split not needed anymore, drop buffer content lock and pin and exit
decide which bucket to split
Attempt to X-lock old bucket number (definitely could fail)
Attempt to X-lock new bucket number (shouldn't fail, but...)
- if above fail, drop locks and exit
+ if above fail, drop locks and pin and exit
update meta page to reflect new number of buckets
- write/release meta page
- release X-lock on page 0
+ mark meta page dirty and release buffer content lock and pin
-- now, accesses to all other buckets can proceed.
Perform actual split of bucket, moving tuples as needed
>> see below about acquiring needed extra space
Release X-locks of old and new buckets
-Note the page zero and metapage locks are not held while the actual tuple
-rearrangement is performed, so accesses to other buckets can proceed in
-parallel; in fact, it's possible for multiple bucket splits to proceed
-in parallel.
+Note the metapage lock is not held while the actual tuple rearrangement is
+performed, so accesses to other buckets can proceed in parallel; in fact,
+it's possible for multiple bucket splits to proceed in parallel.
Split's attempt to X-lock the old bucket number could fail if another
process holds S-lock on it. We do not want to wait if that happens, first
The fourth operation is garbage collection (bulk deletion):
next bucket := 0
- read/sharelock meta page
+ pin metapage and take buffer content lock in exclusive mode
fetch current max bucket number
- release meta page
+ release meta page buffer content lock and pin
while next bucket <= max bucket do
Acquire X lock on target bucket
Scan and remove tuples, compact free space as needed
Release X lock
next bucket ++
end loop
- exclusive-lock meta page
+ pin metapage and take buffer content lock in exclusive mode
check if number of buckets changed
- if so, release lock and return to for-each-bucket loop
+ if so, release content lock and pin and return to for-each-bucket loop
else update metapage tuple count
- write/release meta page
+ mark meta page dirty and release buffer content lock and pin
Note that this is designed to allow concurrent splits. If a split occurs,
tuples relocated into the new bucket will be visited twice by the scan,
Obtaining an overflow page:
- read/exclusive-lock meta page
+ take metapage content lock in exclusive mode
determine next bitmap page number; if none, exit loop
- release meta page lock
- read/exclusive-lock bitmap page
+ release meta page content lock
+ pin bitmap page and take content lock in exclusive mode
search for a free page (zero bit in bitmap)
if found:
set bit in bitmap
- write/release bitmap page
- read/exclusive-lock meta page
+ mark bitmap page dirty and release content lock
+ take metapage buffer content lock in exclusive mode
if first-free-bit value did not change,
- update it and write meta page
- release meta page
+ update it and mark meta page dirty
+ release meta page buffer content lock
return page number
else (not found):
- release bitmap page
+ release bitmap page buffer content lock
loop back to try next bitmap page, if any
-- here when we have checked all bitmap pages; we hold meta excl. lock
extend index to add another overflow page; update meta information
- write/release meta page
+ mark meta page dirty and release buffer content lock
return page number
It is slightly annoying to release and reacquire the metapage lock
delink overflow page from bucket chain
(this requires read/update/write/release of fore and aft siblings)
- read/share-lock meta page
+ pin meta page and take buffer content lock in shared mode
determine which bitmap page contains the free space bit for page
- release meta page
- read/exclusive-lock bitmap page
+ relase meta page buffer content lock
+ pin bitmap page and take buffer content lock in exclusie mode
update bitmap bit
- write/release bitmap page
+ mark bitmap page dirty and release buffer content lock and pin
if page number is less than what we saw as first-free-bit in meta:
- read/exclusive-lock meta page
+ retake meta page buffer content lock in exclusive mode
if page number is still less than first-free-bit,
- update first-free-bit field and write meta page
- release meta page
+ update first-free-bit field and mark meta page dirty
+ release meta page buffer content lock and pin
We have to do it this way because we must clear the bitmap bit before
changing the first-free-bit field (hashm_firstfree). It is possible that
Buffer metabuf;
HashMetaPage metap;
BlockNumber blkno;
+ BlockNumber oldblkno = InvalidBlockNumber;
+ bool retry = false;
Page page;
HashPageOpaque pageopaque;
Size itemsz;
itemsz = MAXALIGN(itemsz); /* be safe, PageAddItem will do this but we
* need to be consistent */
- /*
- * Acquire shared split lock so we can compute the target bucket safely
- * (see README).
- */
- _hash_getlock(rel, 0, HASH_SHARE);
-
/* Read the metapage */
metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
metap = HashPageGetMeta(BufferGetPage(metabuf));
errhint("Values larger than a buffer page cannot be indexed.")));
/*
- * Compute the target bucket number, and convert to block number.
+ * Loop until we get a lock on the correct target bucket.
*/
- bucket = _hash_hashkey2bucket(hashkey,
- metap->hashm_maxbucket,
- metap->hashm_highmask,
- metap->hashm_lowmask);
+ for (;;)
+ {
+ /*
+ * Compute the target bucket number, and convert to block number.
+ */
+ bucket = _hash_hashkey2bucket(hashkey,
+ metap->hashm_maxbucket,
+ metap->hashm_highmask,
+ metap->hashm_lowmask);
- blkno = BUCKET_TO_BLKNO(metap, bucket);
+ blkno = BUCKET_TO_BLKNO(metap, bucket);
- /* release lock on metapage, but keep pin since we'll need it again */
- _hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+ /* Release metapage lock, but keep pin. */
+ _hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
- /*
- * Acquire share lock on target bucket; then we can release split lock.
- */
- _hash_getlock(rel, blkno, HASH_SHARE);
+ /*
+ * If the previous iteration of this loop locked what is still the
+ * correct target bucket, we are done. Otherwise, drop any old lock
+ * and lock what now appears to be the correct bucket.
+ */
+ if (retry)
+ {
+ if (oldblkno == blkno)
+ break;
+ _hash_droplock(rel, oldblkno, HASH_SHARE);
+ }
+ _hash_getlock(rel, blkno, HASH_SHARE);
- _hash_droplock(rel, 0, HASH_SHARE);
+ /*
+ * Reacquire metapage lock and check that no bucket split has taken
+ * place while we were awaiting the bucket lock.
+ */
+ _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+ oldblkno = blkno;
+ retry = true;
+ }
/* Fetch the primary bucket page for the bucket */
buf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BUCKET_PAGE);
/*
* _hash_getlock() -- Acquire an lmgr lock.
*
- * 'whichlock' should be zero to acquire the split-control lock, or the
- * block number of a bucket's primary bucket page to acquire the per-bucket
- * lock. (See README for details of the use of these locks.)
+ * 'whichlock' should the block number of a bucket's primary bucket page to
+ * acquire the per-bucket lock. (See README for details of the use of these
+ * locks.)
*
* 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
*/
uint32 lowmask;
/*
- * Obtain the page-zero lock to assert the right to begin a split (see
- * README).
- *
- * Note: deadlock should be impossible here. Our own backend could only be
- * holding bucket sharelocks due to stopped indexscans; those will not
- * block other holders of the page-zero lock, who are only interested in
- * acquiring bucket sharelocks themselves. Exclusive bucket locks are
- * only taken here and in hashbulkdelete, and neither of these operations
- * needs any additional locks to complete. (If, due to some flaw in this
- * reasoning, we manage to deadlock anyway, it's okay to error out; the
- * index will be left in a consistent state.)
+ * Write-lock the meta page. It used to be necessary to acquire a
+ * heavyweight lock to begin a split, but that is no longer required.
*/
- _hash_getlock(rel, 0, HASH_EXCLUSIVE);
-
- /* Write-lock the meta page */
_hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);
_hash_checkpage(rel, metabuf, LH_META_PAGE);
/* Write out the metapage and drop lock, but keep pin */
_hash_chgbufaccess(rel, metabuf, HASH_WRITE, HASH_NOLOCK);
- /* Release split lock; okay for other splits to occur now */
- _hash_droplock(rel, 0, HASH_EXCLUSIVE);
-
/* Relocate records to the new bucket */
_hash_splitbucket(rel, metabuf, old_bucket, new_bucket,
start_oblkno, start_nblkno,
/* We didn't write the metapage, so just drop lock */
_hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
-
- /* Release split lock */
- _hash_droplock(rel, 0, HASH_EXCLUSIVE);
}
uint32 hashkey;
Bucket bucket;
BlockNumber blkno;
+ BlockNumber oldblkno = InvalidBuffer;
+ bool retry = false;
Buffer buf;
Buffer metabuf;
Page page;
so->hashso_sk_hash = hashkey;
- /*
- * Acquire shared split lock so we can compute the target bucket safely
- * (see README).
- */
- _hash_getlock(rel, 0, HASH_SHARE);
-
/* Read the metapage */
metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
metap = HashPageGetMeta(BufferGetPage(metabuf));
/*
- * Compute the target bucket number, and convert to block number.
+ * Loop until we get a lock on the correct target bucket.
*/
- bucket = _hash_hashkey2bucket(hashkey,
- metap->hashm_maxbucket,
- metap->hashm_highmask,
- metap->hashm_lowmask);
-
- blkno = BUCKET_TO_BLKNO(metap, bucket);
+ for (;;)
+ {
+ /*
+ * Compute the target bucket number, and convert to block number.
+ */
+ bucket = _hash_hashkey2bucket(hashkey,
+ metap->hashm_maxbucket,
+ metap->hashm_highmask,
+ metap->hashm_lowmask);
+
+ blkno = BUCKET_TO_BLKNO(metap, bucket);
+
+ /* Release metapage lock, but keep pin. */
+ _hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
+
+ /*
+ * If the previous iteration of this loop locked what is still the
+ * correct target bucket, we are done. Otherwise, drop any old lock
+ * and lock what now appears to be the correct bucket.
+ */
+ if (retry)
+ {
+ if (oldblkno == blkno)
+ break;
+ _hash_droplock(rel, oldblkno, HASH_SHARE);
+ }
+ _hash_getlock(rel, blkno, HASH_SHARE);
+
+ /*
+ * Reacquire metapage lock and check that no bucket split has taken
+ * place while we were awaiting the bucket lock.
+ */
+ _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_READ);
+ oldblkno = blkno;
+ retry = true;
+ }
/* done with the metapage */
- _hash_relbuf(rel, metabuf);
-
- /*
- * Acquire share lock on target bucket; then we can release split lock.
- */
- _hash_getlock(rel, blkno, HASH_SHARE);
-
- _hash_droplock(rel, 0, HASH_SHARE);
+ _hash_dropbuf(rel, metabuf);
/* Update scan opaque state to show we have lock on the bucket */
so->hashso_bucket = bucket;
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