Re: LogwrtResult contended spinlock

Looking at patterns like this

if (XLogCtl->LogwrtRqst.Write < EndPos)
XLogCtl->LogwrtRqst.Write = EndPos;

It seems possible to implement with

do {
XLogRecPtr currwrite;

currwrite = pg_atomic_read_u64(LogwrtRqst.Write);
if (currwrite > EndPos)
break; // already done by somebody else
if (pg_atomic_compare_exchange_u64(LogwrtRqst.Write,
currwrite, EndPos))
break; // successfully updated
} while (true);

This assumes that LogwrtRqst.Write never goes backwards, so it doesn't
seem good material for a general routine.

This *seems* correct to me, though this is muddy territory to me. Also,
are there better ways to go about this?

--
Álvaro Herrera https://www.2ndQuadrant.com/
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