Re: pg_atomic_compare_exchange_*() and memory barriers - Mailing list pgsql-hackers

On Fri, Mar 7, 2025 at 7:15 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
>
> On Fri, Mar 7, 2025 at 7:07 PM Andres Freund <andres@anarazel.de> wrote:
> > On 2025-03-07 18:39:42 +0200, Alexander Korotkov wrote:
> > > On Fri, Mar 7, 2025 at 6:02 PM Andres Freund <andres@anarazel.de> wrote:
> > > >
> > > > On 2025-03-07 17:47:08 +0200, Alexander Korotkov wrote:
> > > > > While investigating a bug in the patch to get rid of WALBufMappingLock, I
> > > > > found that the surrounding pg_atomic_compare_exchange_u64() fixes the
> > > > > problem for me.
> > > >
> > > > That sounds more likely to be due to slowing down things enough to make a race
> > > > less likely to be hit. Or a compiler bug.
> > > >
> > > >
> > > > > That doesn't feel right because, according to the
> > > > > comments, both pg_atomic_compare_exchange_u32() and
> > > > > pg_atomic_compare_exchange_u64() should provide full memory barrier
> > > > > semantics.  So, I decided to investigate this further.
> > > > >
> > > > > In my case, the implementation of pg_atomic_compare_exchange_u64() is based
> > > > > on __atomic_compare_exchange_n().
> > > > >
> > > > > static inline bool
> > > > > pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
> > > > >                                     uint64 *expected, uint64 newval)
> > > > > {
> > > > >     AssertPointerAlignment(expected, 8);
> > > > >     return __atomic_compare_exchange_n(&ptr->value, expected, newval, false,
> > > > >                                        __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
> > > > > }
> > > > >
> > > > > According to the docs __ATOMIC_SEQ_CST enforces total ordering with *all
> > > > > other __ATOMIC_SEQ_CST operations*.  It only says about other
> > > > > __ATOMIC_SEQ_CST operations; nothing is said about regular reads/writes.
> > > > > This sounds quite far from our comment, promising full barrier semantics,
> > > > > which, in my understanding, means the equivalent of
> > > > > both pg_read_barrier()/pg_write_barrier(), which should barrier *all other
> > > > > read/writes*.
> > > >
> > > > A memory barrier in one process/thread always needs be paired with a barrier
> > > > in another process/thread. It's not enough to have a memory barrier on one
> > > > side but not the other.
> > >
> > > Yes, there surely should be a memory barrier on another side.  But
> > > does __ATOMIC_SEQ_CST works as a barrier for the regular read/write
> > > operations on the same side?
> >
> > Yes, if it's paired with another barrier on the other side. The regular
> > read/write operations are basically protected transitively, due to
> >
> > a) An acquire barrier preventing non-atomic reads/writes in the same thread
> >    from appearing to have been moved to before the barrier
> >
> > b) A release barrier preventing non-atomic reads/writes in the same thread
> >    from appearing to have been moved to after the barrier
> >
> > c) The other thread being guaranteed a) and b) for the other threads'
> >    non-atomic reads/writes depending on the type of barrier
> >
> > d) The atomic value itself being guaranteed to be, well, atomic
>
> Yes, looks good to me.
>
> > > > > This function first checks if LSE instructions are present.  If so,
> > > > > instruction LSE casal is used.  If not (in my case), the loop of
> > > > > ldaxr/stlxr is used instead.  The documentation says both ldaxr/stlxr
> > > > > provides one-way barriers.  Read/writes after ldaxr will be observed after,
> > > > > and read/writes before stlxr will be observed before.  So, a pair of these
> > > > > instructions provides a full memory barrier.  However, if we don't observe
> > > > > the expected value, only ldaxr gets executed.  So, an unsuccessful
> > > > > pg_atomic_compare_exchange_u64() attempt will leave us with a one-way
> > > > > barrier, and that caused a bug in my case.
> > > >
> > > > That has to be a compiler bug.  We specify __ATOMIC_SEQ_CST for both
> > > > success_memorder *and* failure_memorder.
> > > >
> > > > What compiler & version is this?
> > >
> > > I've tried and got the same results with two compilers.
> > > gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
> > > Ubuntu clang version 19.1.1 (1ubuntu1~24.04.2)
> >
> > Thinking more about it I wonder if the behaviour of not doing a release in
> > case the atomic fails *might* arguably actually be correct - if the
> > compare-exchange fails, there's nothing that the non-atomic values could be
> > ordered against.
> >
> > 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).
>
> 1. p1 executes l1
> 2. p2 executes l2 with success
> 3. p1 executes l2 with failure
> 4. p2 execute l3, but doesn't see the results of step 1, because 3 didn't provide enough of memory barrier

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

Note that step 2 would be successful if it would be after step 3.

------
Regards,
Alexander Korotkov
Supabase