* so it's a plain spinlock. The other locks are held longer (potentially
* over I/O operations), so we use LWLocks for them. These locks are:
*
+ * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
+ * It is only held while initializing and changing the mapping. If the
+ * contents of the buffer being replaced haven't been written yet, the mapping
+ * lock is released while the write is done, and reacquired afterwards.
+ *
* WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
* XLogFlush).
*
pg_atomic_uint64 logFlushResult; /* last byte + 1 flushed */
/*
- * First initialized page in the cache (first byte position).
- */
- XLogRecPtr InitializedFrom;
-
- /*
- * Latest reserved for initialization page in the cache (last byte
- * position + 1).
+ * Latest initialized page in the cache (last byte position + 1).
*
- * To change the identity of a buffer, you need to advance
- * InitializeReserved first. To change the identity of a buffer that's
+ * To change the identity of a buffer (and InitializedUpTo), you need to
+ * hold WALBufMappingLock. To change the identity of a buffer that's
* still dirty, the old page needs to be written out first, and for that
* you need WALWriteLock, and you need to ensure that there are no
* in-progress insertions to the page by calling
* WaitXLogInsertionsToFinish().
*/
- pg_atomic_uint64 InitializeReserved;
-
- /*
- * Latest initialized page in the cache (last byte position + 1).
- *
- * InitializedUpTo is updated after the buffer initialization. After
- * update, waiters got notification using InitializedUpToCondVar.
- */
- pg_atomic_uint64 InitializedUpTo;
- ConditionVariable InitializedUpToCondVar;
+ XLogRecPtr InitializedUpTo;
/*
* These values do not change after startup, although the pointed-to pages
- * and xlblocks values certainly do. xlblocks values are changed
- * lock-free according to the check for the xlog write position and are
- * accompanied by changes of InitializeReserved and InitializedUpTo.
+ * and xlblocks values certainly do. xlblocks values are protected by
+ * WALBufMappingLock.
*/
char *pages; /* buffers for unwritten XLOG pages */
pg_atomic_uint64 *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
* fullPageWrites from changing until the insertion is finished.
*
* Step 2 can usually be done completely in parallel. If the required WAL
- * page is not initialized yet, you have to go through AdvanceXLInsertBuffer,
- * which will ensure it is initialized. But the WAL writer tries to do that
- * ahead of insertions to avoid that from happening in the critical path.
+ * page is not initialized yet, you have to grab WALBufMappingLock to
+ * initialize it, but the WAL writer tries to do that ahead of insertions
+ * to avoid that from happening in the critical path.
*
*----------
*/
XLogRecPtr NewPageEndPtr = InvalidXLogRecPtr;
XLogRecPtr NewPageBeginPtr;
XLogPageHeader NewPage;
- XLogRecPtr ReservedPtr;
int npages pg_attribute_unused() = 0;
- /*
- * We must run the loop below inside the critical section as we expect
- * XLogCtl->InitializedUpTo to eventually keep up. The most of callers
- * already run inside the critical section. Except for WAL writer, which
- * passed 'opportunistic == true', and therefore we don't perform
- * operations that could error out.
- *
- * Start an explicit critical section anyway though.
- */
- Assert(CritSectionCount > 0 || opportunistic);
- START_CRIT_SECTION();
+ LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
- /*--
- * Loop till we get all the pages in WAL buffer before 'upto' reserved for
- * initialization. Multiple process can initialize different buffers with
- * this loop in parallel as following.
- *
- * 1. Reserve page for initialization using XLogCtl->InitializeReserved.
- * 2. Initialize the reserved page.
- * 3. Attempt to advance XLogCtl->InitializedUpTo,
+ /*
+ * Now that we have the lock, check if someone initialized the page
+ * already.
*/
- ReservedPtr = pg_atomic_read_u64(&XLogCtl->InitializeReserved);
- while (upto >= ReservedPtr || opportunistic)
+ while (upto >= XLogCtl->InitializedUpTo || opportunistic)
{
- Assert(ReservedPtr % XLOG_BLCKSZ == 0);
+ nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);
/*
- * Get ending-offset of the buffer page we need to replace.
- *
- * We don't lookup into xlblocks, but rather calculate position we
- * must wait to be written. If it was written, xlblocks will have this
- * position (or uninitialized)
+ * Get ending-offset of the buffer page we need to replace (this may
+ * be zero if the buffer hasn't been used yet). Fall through if it's
+ * already written out.
*/
- if (ReservedPtr + XLOG_BLCKSZ > XLogCtl->InitializedFrom + XLOG_BLCKSZ * XLOGbuffers)
- OldPageRqstPtr = ReservedPtr + XLOG_BLCKSZ - (XLogRecPtr) XLOG_BLCKSZ * XLOGbuffers;
- else
- OldPageRqstPtr = InvalidXLogRecPtr;
-
- if (LogwrtResult.Write < OldPageRqstPtr && opportunistic)
+ OldPageRqstPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
+ if (LogwrtResult.Write < OldPageRqstPtr)
{
/*
- * If we just want to pre-initialize as much as we can without
- * flushing, give up now.
+ * Nope, got work to do. If we just want to pre-initialize as much
+ * as we can without flushing, give up now.
*/
- upto = ReservedPtr - 1;
- break;
- }
-
- /*
- * Attempt to reserve the page for initialization. Failure means that
- * this page got reserved by another process.
- */
- if (!pg_atomic_compare_exchange_u64(&XLogCtl->InitializeReserved,
- &ReservedPtr,
- ReservedPtr + XLOG_BLCKSZ))
- continue;
-
- /*
- * Wait till page gets correctly initialized up to OldPageRqstPtr.
- */
- nextidx = XLogRecPtrToBufIdx(ReservedPtr);
- while (pg_atomic_read_u64(&XLogCtl->InitializedUpTo) < OldPageRqstPtr)
- ConditionVariableSleep(&XLogCtl->InitializedUpToCondVar, WAIT_EVENT_WAL_BUFFER_INIT);
- ConditionVariableCancelSleep();
- Assert(pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]) == OldPageRqstPtr);
-
- /* Fall through if it's already written out. */
- if (LogwrtResult.Write < OldPageRqstPtr)
- {
- /* Nope, got work to do. */
+ if (opportunistic)
+ break;
/* Advance shared memory write request position */
SpinLockAcquire(&XLogCtl->info_lck);
RefreshXLogWriteResult(LogwrtResult);
if (LogwrtResult.Write < OldPageRqstPtr)
{
+ /*
+ * Must acquire write lock. Release WALBufMappingLock first,
+ * to make sure that all insertions that we need to wait for
+ * can finish (up to this same position). Otherwise we risk
+ * deadlock.
+ */
+ LWLockRelease(WALBufMappingLock);
+
WaitXLogInsertionsToFinish(OldPageRqstPtr);
LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
*/
pgstat_report_fixed = true;
}
+ /* Re-acquire WALBufMappingLock and retry */
+ LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
+ continue;
}
}
* Now the next buffer slot is free and we can set it up to be the
* next output page.
*/
- NewPageBeginPtr = ReservedPtr;
+ NewPageBeginPtr = XLogCtl->InitializedUpTo;
NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;
+ Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);
+
NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);
/*
*/
pg_write_barrier();
- /*-----
- * Update the value of XLogCtl->xlblocks[nextidx] and try to advance
- * XLogCtl->InitializedUpTo in a lock-less manner.
- *
- * First, let's provide a formal proof of the algorithm. Let it be 'n'
- * process with the following variables in shared memory:
- * f - an array of 'n' boolean flags,
- * v - atomic integer variable.
- *
- * Also, let
- * i - a number of a process,
- * j - local integer variable,
- * CAS(var, oldval, newval) - compare-and-swap atomic operation
- * returning true on success,
- * write_barrier()/read_barrier() - memory barriers.
- *
- * The pseudocode for each process is the following.
- *
- * j := i
- * f[i] := true
- * write_barrier()
- * while CAS(v, j, j + 1):
- * j := j + 1
- * read_barrier()
- * if not f[j]:
- * break
- *
- * Let's prove that v eventually reaches the value of n.
- * 1. Prove by contradiction. Assume v doesn't reach n and stucks
- * on k, where k < n.
- * 2. Process k attempts CAS(v, k, k + 1). 1). If, as we assumed, v
- * gets stuck at k, then this CAS operation must fail. Therefore,
- * v < k when process k attempts CAS(v, k, k + 1).
- * 3. If, as we assumed, v gets stuck at k, then the value k of v
- * must be achieved by some process m, where m < k. The process
- * m must observe f[k] == false. Otherwise, it will later attempt
- * CAS(v, k, k + 1) with success.
- * 4. Therefore, corresponding read_barrier() (while j == k) on
- * process m reached before write_barrier() of process k. But then
- * process k attempts CAS(v, k, k + 1) after process m successfully
- * incremented v to k, and that CAS operation must succeed.
- * That leads to a contradiction. So, there is no such k (k < n)
- * where v gets stuck. Q.E.D.
- *
- * To apply this proof to the code below, we assume
- * XLogCtl->InitializedUpTo will play the role of v with XLOG_BLCKSZ
- * granularity. We also assume setting XLogCtl->xlblocks[nextidx] to
- * NewPageEndPtr to play the role of setting f[i] to true. Also, note
- * that processes can't concurrently map different xlog locations to
- * the same nextidx because we previously requested that
- * XLogCtl->InitializedUpTo >= OldPageRqstPtr. So, a xlog buffer can
- * be taken for initialization only once the previous initialization
- * takes effect on XLogCtl->InitializedUpTo.
- */
-
pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], NewPageEndPtr);
-
- pg_write_barrier();
-
- while (pg_atomic_compare_exchange_u64(&XLogCtl->InitializedUpTo, &NewPageBeginPtr, NewPageEndPtr))
- {
- NewPageBeginPtr = NewPageEndPtr;
- NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;
- nextidx = XLogRecPtrToBufIdx(NewPageBeginPtr);
-
- pg_read_barrier();
-
- if (pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]) != NewPageEndPtr)
- {
- /*
- * Page at nextidx wasn't initialized yet, so we can't move
- * InitializedUpto further. It will be moved by backend which
- * will initialize nextidx.
- */
- ConditionVariableBroadcast(&XLogCtl->InitializedUpToCondVar);
- break;
- }
- }
+ XLogCtl->InitializedUpTo = NewPageEndPtr;
npages++;
}
-
- END_CRIT_SECTION();
-
- /*
- * All the pages in WAL buffer before 'upto' were reserved for
- * initialization. However, some pages might be reserved by concurrent
- * processes. Wait till they finish initialization.
- */
- while (upto >= pg_atomic_read_u64(&XLogCtl->InitializedUpTo))
- ConditionVariableSleep(&XLogCtl->InitializedUpToCondVar, WAIT_EVENT_WAL_BUFFER_INIT);
- ConditionVariableCancelSleep();
-
- pg_read_barrier();
+ LWLockRelease(WALBufMappingLock);
#ifdef WAL_DEBUG
if (XLOG_DEBUG && npages > 0)
pg_atomic_init_u64(&XLogCtl->logWriteResult, InvalidXLogRecPtr);
pg_atomic_init_u64(&XLogCtl->logFlushResult, InvalidXLogRecPtr);
pg_atomic_init_u64(&XLogCtl->unloggedLSN, InvalidXLogRecPtr);
-
- pg_atomic_init_u64(&XLogCtl->InitializeReserved, InvalidXLogRecPtr);
- pg_atomic_init_u64(&XLogCtl->InitializedUpTo, InvalidXLogRecPtr);
- ConditionVariableInit(&XLogCtl->InitializedUpToCondVar);
}
/*
memset(page + len, 0, XLOG_BLCKSZ - len);
pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx], endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
- pg_atomic_write_u64(&XLogCtl->InitializedUpTo, endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
- XLogCtl->InitializedFrom = endOfRecoveryInfo->lastPageBeginPtr;
+ XLogCtl->InitializedUpTo = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
}
else
{
* let the first attempt to insert a log record to initialize the next
* buffer.
*/
- pg_atomic_write_u64(&XLogCtl->InitializedUpTo, EndOfLog);
- XLogCtl->InitializedFrom = EndOfLog;
+ XLogCtl->InitializedUpTo = EndOfLog;
}
- pg_atomic_write_u64(&XLogCtl->InitializeReserved, pg_atomic_read_u64(&XLogCtl->InitializedUpTo));
/*
* Update local and shared status. This is OK to do without any locks
projects / thirdparty / postgresql.git / commitdiff
