Re: slru.c race condition (was Re: [HACKERS] TRAP: FailedAssertion("!((itemid)->lp_flags & 0x01)",)

I wrote:
> I think it would be possible to fix it in a less invasive way by taking
> and releasing the ControlLock an extra time in SimpleLruReadPage and
> SimpleLruWritePage.  What's indeterminate about that is the performance
> cost.

Attached is an alternative patch that does it this way.  I realized that
we could use LWLockConditionalAcquire to usually avoid any extra lock
traffic, so the performance cost may be negligible except under the very
heaviest of loads.  I still like the bigger patch for 8.2 and forward,
because it's a lot cleaner, but this seems like a credible alternative
for 8.1 and back branches.

Comments?

            regards, tom lane