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Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_START.

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Blocking checkpoint phase 2 requires MarkBufferDirty() and
BUFFER_LOCK_EXCLUSIVE; neither suffices by itself.  transam/README documents
this, citing SyncOneBuffer().  Update the DELAY_CHKPT_START documentation to
say this.  Expand the heap_inplace_update_and_unlock() comment that cites
XLogSaveBufferForHint() as precedent, since heap_inplace_update_and_unlock()
could have opted not to use DELAY_CHKPT_START.

Commit 8e7e672cda added DELAY_CHKPT_START to
heap_inplace_update_and_unlock().  Since commit
bc6bad8857 reverted it in non-master branches,
no back-patch.

Discussion: https://postgr.es/m/20250406180054.26.nmisch@google.com
This commit is contained in:
Noah Misch
2025-04-20 12:00:17 -07:00
parent 714bd9e3a7
commit 8180136652
2 changed files with 15 additions and 7 deletions
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14
src/backend/access/heap/heapam.c
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@ -6507,9 +6507,17 @@ heap_inplace_update_and_unlock(Relation relation,
* [crash]
* [recovery restores datfrozenxid w/o relfrozenxid]
*
* Like in MarkBufferDirtyHint() subroutine XLogSaveBufferForHint(), copy
* the buffer to the stack before logging. Here, that facilitates a FPI
* of the post-mutation block before we accept other sessions seeing it.
* Mimic MarkBufferDirtyHint() subroutine XLogSaveBufferForHint().
* Specifically, use DELAY_CHKPT_START, and copy the buffer to the stack.
* The stack copy facilitates a FPI of the post-mutation block before we
* accept other sessions seeing it. DELAY_CHKPT_START allows us to
* XLogInsert() before MarkBufferDirty(). Since XLogSaveBufferForHint()
* can operate under BUFFER_LOCK_SHARED, it can't avoid DELAY_CHKPT_START.
* This function, however, likely could avoid it with the following order
* of operations: MarkBufferDirty(), XLogInsert(), memcpy(). Opt to use
* DELAY_CHKPT_START here, too, as a way to have fewer distinct code
* patterns to analyze. Inplace update isn't so frequent that it should
* pursue the small optimization of skipping DELAY_CHKPT_START.
*/
Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
START_CRIT_SECTION();

8
src/include/storage/proc.h
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@ -110,10 +110,10 @@ extern PGDLLIMPORT int FastPathLockGroupsPerBackend;
* is inserted prior to the new redo point, the corresponding data changes will
* also be flushed to disk before the checkpoint can complete. (In the
* extremely common case where the data being modified is in shared buffers
* and we acquire an exclusive content lock on the relevant buffers before
* writing WAL, this mechanism is not needed, because phase 2 will block
* until we release the content lock and then flush the modified data to
* disk.)
* and we acquire an exclusive content lock and MarkBufferDirty() on the
* relevant buffers before writing WAL, this mechanism is not needed, because
* phase 2 will block until we release the content lock and then flush the
* modified data to disk. See transam/README and SyncOneBuffer().)
*
* Setting DELAY_CHKPT_COMPLETE prevents the system from moving from phase 2
* to phase 3. This is useful if we are performing a WAL-logged operation that