Re: Yes, WaitLatch is vulnerable to weak-memory-ordering bugs

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Yes, WaitLatch is vulnerable to weak-memory-ordering bugs
Date: 2011-08-08 16:45:07
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On 08.08.2011 19:39, Tom Lane wrote:
> Robert Haas<robertmhaas(at)gmail(dot)com> writes:
>> On the flip side, I'm not sure that anyone ever really expected that a
>> latch could be safely used this way. Normally, I'd expect the flag to
>> be some piece of state protected by an LWLock, and I think that ought
>> to be OK provided that the lwlock is released before setting or
>> checking the latch. Maybe we should try to document the contract here
>> a little better; I think it's just that there must be a full memory
>> barrier (such as LWLockRelease) in both processes involved in the
>> interaction.
> Heikki argued the same thing in
> but I don't think anyone has actually done the legwork to verify that
> current uses are safe. So [ ... warms up grep ... ]

Thanks for double-checking.

> (3) I'm not going to hold still for not inserting a memory barrier
> into SetLatch, once we have the code. Unsubstantiated performance
> worries are not a sufficient argument not to --- as a wise man once
> said, you can make the code arbitrarily fast if it doesn't have to
> give the right answer.

I agree we definitely should add the memory barrier instruction there
once we have them.

> (4) In the meantime, we'd better document that it's caller's
> responsibility to ensure that the flag variable is adequately
> protected by locking. I think SyncRepWaitForLSN could use a warning
> comment, too. I will go do that.

Ok, thanks.

Heikki Linnakangas

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