Re: Memory ordering issue in LWLockRelease, WakeupWaiters, WALInsertSlotRelease

On 2014-02-14 20:23:32 +0400, knizhnik wrote:
> >>we'll trade correctness for cleanliness if we continue to reset lwWaitLink
> >>without protecting against the race. That's a bit of a weird trade-off to make.
> >
> >It's not just cleanliness, it's being able to actually debug crashes.
>
>
> Frankly speaking I do not understand why elimination of resetting of lwWaitLink was considered to be bad idea.
> Resetting this pointer to NULL will not help much to analyze crash dumps, because right now it is possible that lwWaitLink==NULL but process in waiting for a lock and linked in the list
> (if it is the last element of the list). So the fact that lwWaitLink==NULL actually gives not so much useful information.

At the moment if you connect to a live pg or a crash dump, investigating
the wait links allows you to somewhat sensibly determine which backends
are waiting for a lock and whether they are part of a queue. If they
aren't reset anymore that will tell you nothing, so you'll need to
connect to all pg instances to debug issues.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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