Date: Sat, 10 Jun 2017 23:24:48 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Michael Paquier <michael.paquier@gmail.com>
Cc: wangchuanting <wangchuanting@huawei.com>,
	PostgreSQL mailing lists <pgsql-bugs@postgresql.org>
Subject: Re: Re: BUG #14680: startup process on standby encounter a
 deadlock of TwoPhaseStateLock when redo 2PC xlog
Message-ID: <20170611032448.f2da5epz5pnbfmmj@alvherre.pgsql>
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Alvaro Herrera wrote:

For some reason I sent a preliminary version of the email I was writing.
Some additional thoughts that were supposed to be in there:

> I also agree that restoreTwoPhaseData doesn't need to hold the lock,
> since we don't expect anything running concurrently, but that it's okay
> to hold it and makes the whole thing simpler.

It's okay to hold the lock for the whole duration of the function.

> We could try to apply an equivalent rationale to
> RecoverPreparedTransactions: even though we have already been running in
> HOT standby mode for a while, there's no possibility of concurrent
> activity either: since we exited recovery, master cannot write any more
> 2PC xacts, and we haven't yet set the flag that lets open sessions write
> WAL.  However, it seems mildly dangerous to run the bottom sections of
> the loop with the lock held, since it acquires other lwlocks and
> generally does a lot of crap.

Therefore, let's adopt your idea of acquiring the lock only for the
specific low-level calls that require it.  But the comment atop the loop
needs a lot of work to explain why it's doing that (i.e. how come it's
reading TwoPhaseState without a lock), as in my proposed patch.

(At first, I didn't really like this very much and wanted to add more
locking to that function, but it turned quite messy, so I back-tracked).

Thanks

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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