pg_atomic_exchange_u32() in ProcArrayGroupClearXid()

Hi!

While investigating ProcArrayGroupClearXid() code I wonder why do we have
this loop instead of plain pg_atomic_exchange_u32() call?  Is it
intentional?

/*
* Now that we've got the lock, clear the list of processes waiting for
* group XID clearing, saving a pointer to the head of the list.  Trying
* to pop elements one at a time could lead to an ABA problem.
*/
while (true)
{
nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
   &nextidx,
   INVALID_PGPROCNO))
break;
}

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Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company