Use 64-bit atomics for xlblocks array elements.
In preparation for reading the contents of WAL buffers without a lock. Also, avoids the previously-needed comment in GetXLogBuffer() explaining why it's safe from torn reads. Author: Bharath Rupireddy Discussion: https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.com Reviewed-by: Andres Freund
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@ -501,7 +501,7 @@ typedef struct XLogCtlData
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* WALBufMappingLock.
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*/
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char *pages; /* buffers for unwritten XLOG pages */
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XLogRecPtr *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
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pg_atomic_uint64 *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
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int XLogCacheBlck; /* highest allocated xlog buffer index */
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/*
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@ -1636,20 +1636,16 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
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* out to disk and evicted, and the caller is responsible for making sure
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* that doesn't happen.
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*
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* However, we don't hold a lock while we read the value. If someone has
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* just initialized the page, it's possible that we get a "torn read" of
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* the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
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* that case we will see a bogus value. That's ok, we'll grab the mapping
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* lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
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* the page we're looking for. But it means that when we do this unlocked
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* read, we might see a value that appears to be ahead of the page we're
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* looking for. Don't PANIC on that, until we've verified the value while
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* holding the lock.
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* We don't hold a lock while we read the value. If someone is just about
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* to initialize or has just initialized the page, it's possible that we
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* get InvalidXLogRecPtr. That's ok, we'll grab the mapping lock (in
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* AdvanceXLInsertBuffer) and retry if we see anything other than the page
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* we're looking for.
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*/
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expectedEndPtr = ptr;
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expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;
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endptr = XLogCtl->xlblocks[idx];
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endptr = pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
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if (expectedEndPtr != endptr)
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{
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XLogRecPtr initializedUpto;
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@ -1680,7 +1676,7 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
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WALInsertLockUpdateInsertingAt(initializedUpto);
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AdvanceXLInsertBuffer(ptr, tli, false);
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endptr = XLogCtl->xlblocks[idx];
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endptr = pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
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if (expectedEndPtr != endptr)
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elog(PANIC, "could not find WAL buffer for %X/%X",
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@ -1867,7 +1863,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
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* be zero if the buffer hasn't been used yet). Fall through if it's
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* already written out.
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*/
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OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
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OldPageRqstPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
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if (LogwrtResult.Write < OldPageRqstPtr)
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{
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/*
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@ -1989,8 +1985,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
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*/
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pg_write_barrier();
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*((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;
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pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], NewPageEndPtr);
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XLogCtl->InitializedUpTo = NewPageEndPtr;
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npages++;
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@ -2208,7 +2203,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
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* if we're passed a bogus WriteRqst.Write that is past the end of the
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* last page that's been initialized by AdvanceXLInsertBuffer.
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*/
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XLogRecPtr EndPtr = XLogCtl->xlblocks[curridx];
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XLogRecPtr EndPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[curridx]);
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if (LogwrtResult.Write >= EndPtr)
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elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
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@ -4632,7 +4627,7 @@ XLOGShmemSize(void)
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/* WAL insertion locks, plus alignment */
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size = add_size(size, mul_size(sizeof(WALInsertLockPadded), NUM_XLOGINSERT_LOCKS + 1));
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/* xlblocks array */
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size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
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size = add_size(size, mul_size(sizeof(pg_atomic_uint64), XLOGbuffers));
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/* extra alignment padding for XLOG I/O buffers */
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size = add_size(size, Max(XLOG_BLCKSZ, PG_IO_ALIGN_SIZE));
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/* and the buffers themselves */
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@ -4710,10 +4705,13 @@ XLOGShmemInit(void)
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* needed here.
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*/
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allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
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XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
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memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
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allocptr += sizeof(XLogRecPtr) * XLOGbuffers;
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XLogCtl->xlblocks = (pg_atomic_uint64 *) allocptr;
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allocptr += sizeof(pg_atomic_uint64) * XLOGbuffers;
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for (i = 0; i < XLOGbuffers; i++)
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{
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pg_atomic_init_u64(&XLogCtl->xlblocks[i], InvalidXLogRecPtr);
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}
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/* WAL insertion locks. Ensure they're aligned to the full padded size */
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allocptr += sizeof(WALInsertLockPadded) -
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@ -5750,7 +5748,7 @@ StartupXLOG(void)
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memcpy(page, endOfRecoveryInfo->lastPage, len);
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memset(page + len, 0, XLOG_BLCKSZ - len);
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XLogCtl->xlblocks[firstIdx] = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
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pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx], endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
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XLogCtl->InitializedUpTo = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
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}
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else
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