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Commit6b93fcd

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Avoid atomic operation in MarkLocalBufferDirty().
The recent patch to make Pin/UnpinBuffer lockfree in the hotpath (4835458), accidentally used pg_atomic_fetch_or_u32() inMarkLocalBufferDirty(). Other code operating on local buffers wascareful to only use pg_atomic_read/write_u32 which just read/write frommemory; to avoid unnecessary overhead.On its own that'd just make MarkLocalBufferDirty() slightly lessefficient, but in addition InitLocalBuffers() doesn't callpg_atomic_init_u32() - thus the spinlock fallback for the atomicoperations isn't initialized. That in turn caused, as reported by Tom,buildfarm animal gaur to fail. As those errors are actually usefulagainst this type of error, continue to omit - intentionally this time -initialization of the atomic variable.In addition, add an explicit note about only using pg_atomic_read/writeon local buffers's state to BufferDesc's description.Reported-By: Tom LaneDiscussion: 1881.1460431476@sss.pgh.pa.us
1 parent95ef43c commit6b93fcd

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2 files changed

+16
-3
lines changed

2 files changed

+16
-3
lines changed

‎src/backend/storage/buffer/localbuf.c

Lines changed: 12 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -294,10 +294,14 @@ MarkLocalBufferDirty(Buffer buffer)
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bufHdr=GetLocalBufferDescriptor(bufid);
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297-
buf_state=pg_atomic_fetch_or_u32(&bufHdr->state,BM_DIRTY);
297+
buf_state=pg_atomic_read_u32(&bufHdr->state);
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if (!(buf_state&BM_DIRTY))
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pgBufferUsage.local_blks_dirtied++;
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buf_state |=BM_DIRTY;
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304+
pg_atomic_write_u32(&bufHdr->state,buf_state);
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}
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/*
@@ -431,6 +435,13 @@ InitLocalBuffers(void)
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* is -1.)
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*/
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buf->buf_id=-i-2;
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/*
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* Intentionally do not initialize the buffer's atomic variable
441+
* (besides zeroing the underlying memory above). That way we get
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* errors on platforms without atomics, if somebody (re-)introduces
443+
* atomic operations for local buffers.
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*/
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}
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/* Create the lookup hash table */

‎src/include/storage/buf_internals.h

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -165,8 +165,10 @@ typedef struct buftag
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* wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER. At present,
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* there can be only one such waiter per buffer.
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*
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* We use this same struct for local buffer headers, but the lock fields
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* are not used and not all of the flag bits are useful either.
168+
* We use this same struct for local buffer headers, but the locks are not
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* used and not all of the flag bits are useful either. To avoid unnecessary
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* overhead, manipulations of the state field should be done without actual
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* atomic operations (i.e. only pg_atomic_read/write).
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*
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* Be careful to avoid increasing the size of the struct when adding or
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* reordering members. Keeping it below 64 bytes (the most common CPU

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