Re: INSERT...ON DUPLICATE KEY LOCK FOR UPDATE

On Fri, Sep 20, 2013 at 5:48 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
>> ProcLockWakeup() only wakes as many waiters from the head of the queue
>> as can all be granted the lock without any conflicts. So I don't
>> think there is a race condition in that path.
>
> Right, but what about XactLockTableWait() itself? It only acquires a
> ShareLock on the xid of the got-there-first inserter that potentially
> hasn't yet committed/aborted. There will be no conflicts between
> multiple second-chance-seeking blockers trying to acquire this lock
> concurrently, and so in fact there is (what I guess you'd consider to
> be) a race condition in the current btree insertion code.

I should add: README.tuplock says the following:

"""

The protocol for waiting for a tuple-level lock is really

LockTuple()
XactLockTableWait()
mark tuple as locked by me
UnlockTuple()

When there are multiple waiters, arbitration of who is to get the lock next
is provided by LockTuple().

"""

So because this isn't a tuple-level lock - it's really a value-level
lock - LockTuple() is not called by the btree code at all, and so
arbitration of who gets the lock is, as I've said, essentially
undefined.

--
Peter Geoghegan