On 01/31/2017 05:03 PM, Konstantin Knizhnik wrote:
One more assertion failure:

ExceptionalCondition(conditionName = "!(OldPageRqstPtr <=
XLogCtl->InitializedUpTo)", errorType = "FailedAssertion", fileName =
"xlog.c", lineNumber = 1887), line 54 in "assert.c"

(dbx) p OldPageRqstPtr
(dbx) p XLogCtl->InitializedUpTo
(dbx) p InitializedUpTo

I slightly modify xlog.c code - store value of XLogCtl->InitializedUpTo
in local variable:

  1870         LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
  1872         /*
  1873          * Now that we have the lock, check if someone
initialized the page
  1874          * already.
  1875          */
  1876         while (upto >= XLogCtl->InitializedUpTo || opportunistic)
  1877         {
  1878                 XLogRecPtr InitializedUpTo =
  1879                 nextidx = XLogRecPtrToBufIdx(InitializedUpTo);
  1881                 /*
  1882                  * Get ending-offset of the buffer page we need
to replace (this may
  1883                  * be zero if the buffer hasn't been used yet).
Fall through if it's
  1884                  * already written out.
  1885                  */
  1886                 OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
  1887                 Assert(OldPageRqstPtr <= XLogCtl->InitializedUpTo);

And, as you can see,  XLogCtl->InitializedUpTo is not equal to saved
value InitializedUpTo.
But we are under exclusive WALBufMappingLock and InitializedUpTo is
updated only under this lock.
So it means that LW-locks doesn't work!

Yeah, so it seems. XLogCtl->InitializeUpTo is quite clearly protected by the WALBufMappingLock. All references to it (after StartupXLog) happen while holding the lock.

Can you get the assembly output of the AdvanceXLInsertBuffer() function? I wonder if the compiler is rearranging things so that XLogCtl->InitializedUpTo is fetched before the LWLockAcquire call. Or should there be a memory barrier instruction somewhere in LWLockAcquire?

- Heikki

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