Subject: Re: race condition when writing pg_control
To: Michael Paquier <michael@paquier.xyz>,
 "Bossart, Nathan" <bossartn@amazon.com>
Cc: Thomas Munro <thomas.munro@gmail.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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 <637e17dd-e90b-0dab-f215-dbbe9c6c726a@oss.nttdata.com>
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 <20200527071053.GD103662@paquier.xyz>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <fc796148-7d63-47bb-e91d-e09b62a502e9@oss.nttdata.com>
Date: Fri, 29 May 2020 16:24:12 +0900
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On 2020/05/27 16:10, Michael Paquier wrote:
> On Tue, May 26, 2020 at 07:30:54PM +0000, Bossart, Nathan wrote:
>> While an assertion in UpdateControlFile() would not have helped us
>> catch the problem I initially reported, it does seem worthwhile to add
>> it.  I have attached a patch that adds this assertion and also
>> attempts to fix XLogReportParameters().  Since there is only one place
>> where we feel it is safe to call UpdateControlFile() without a lock, I
>> just changed it to take the lock.  I don't think this adds any sort of
>> significant contention risk, and IMO it is a bit cleaner than the
>> boolean flag.
> 
> Let's see what Fujii-san and Thomas think about that.  I'd rather
> avoid taking a lock here because we don't need it and because it makes
> things IMO confusing with the beginning of StartupXLOG() where a lot
> of the fields are read, even if we go without this extra assertion.

I have no strong opinion about this, but I tend to agree with Michael here.
  
>> For the XLogReportParameters() fix, I simply added an exclusive lock
>> acquisition for the portion that updates the values in shared memory
>> and calls UpdateControlFile().  IIUC the first part of this function
>> that accesses several ControlFile values should be safe, as none of
>> them can be updated after server start.
> 
> They can get updated when replaying a XLOG_PARAMETER_CHANGE record.
> But you are right as all of this happens in the startup process, so
> your patch looks right to me here.

LGTM.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION