On Fri, Mar 7, 2025 at 7:38 PM Andres Freund <andres@anarazel.de> wrote:
> On 2025-03-07 19:15:23 +0200, Alexander Korotkov wrote:
> > On Fri, Mar 7, 2025 at 7:07 PM Andres Freund <andres@anarazel.de> wrote:
> > > What is the access pattern and the observed problems with it that made you
> > > look at the disassembly?
> >
> > Check this code.
> >
> > l1: pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], NewPageEndPtr);
>
> > /*
> > * Try to advance XLogCtl->InitializedUpTo.
> > *
> > * If the CAS operation failed, then some of previous pages are not
> > * initialized yet, and this backend gives up.
> > *
> > * Since initializer of next page might give up on advancing of
> > * InitializedUpTo, this backend have to attempt advancing until it
> > * find page "in the past" or concurrent backend succeeded at
> > * advancing. When we finish advancing XLogCtl->InitializedUpTo, we
> > * notify all the waiters with XLogCtl->InitializedUpToCondVar.
> > */
> > l2: while (pg_atomic_compare_exchange_u64(&XLogCtl->InitializedUpTo,
> > &NewPageBeginPtr, NewPageEndPtr))
> > {
> > NewPageBeginPtr = NewPageEndPtr;
> > NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;
> > nextidx = XLogRecPtrToBufIdx(NewPageBeginPtr);
> >
> > l3: if (pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]) !=
> > NewPageEndPtr)
> > {
> > /*
> > * Page at nextidx wasn't initialized yet, so we cann't move
> > * InitializedUpto further. It will be moved by backend
> > which
> > * will initialize nextidx.
> > */
> >
> > ConditionVariableBroadcast(&XLogCtl->InitializedUpToCondVar);
> > break;
> > }
> > }
> >
> > Consider the following execution order with process 1 (p1) and process 2
> > (p2).
>
> On 2025-03-07 19:24:39 +0200, Alexander Korotkov wrote:
> > Sorry, I messed this up.
> > The correct sequence is following.
> >
> > 1. p1 executes l1
> > 2. p1 executes l2 with failure
> > 3. p2 executes l2 with success
> > 4. p2 execute l3, but doesn't see the results of step 1, because 3
> > didn't provide enough of memory barrier
>
> Did you mean because 2) didn't provide enough of a memory barrier? Because 3)
> does, right?
Yes, exactly.
> You could get in exactly same the situation if the p1 is scheduled out by the
> OS after step 1, no?
No. In that case, p1 will execute l2 with success. p1 executes l2
with failure only because it goes before p2 executes l2.
------
Regards,
Alexander Korotkov
Supabase
