Re: LWLock deadlock and gdb advice

On 06/30/2015 11:24 PM, Andres Freund wrote:
> On 2015-06-30 22:19:02 +0300, Heikki Linnakangas wrote:
>>> Hm. Right. A recheck of the value after the queuing should be sufficient
>>> to fix? That's how we deal with the exact same scenarios for the normal
>>> lock state, so that shouldn't be very hard to add.
>>
>> Yeah. It's probably more efficient to not release the spinlock between
>> checking the value and LWLockQueueSelf() though.
>
> Right now LWLockQueueSelf() takes spinlocks etc itself, and is used that
> way in a bunch of callsites... So that'd be harder. Additionally I'm
> planning to get rid of the spinlocks around queuing (they show up as
> bottlenecks in contended workloads), so it seems more future proof not
> to assume that either way. On top of that I think we should, when
> available (or using the same type of fallback as for 32bit?), use 64 bit
> atomics for the var anyway.
>
>> I'll take a stab at fixing this tomorrow.
>
> Thanks! Do you mean both or "just" the second issue?

Both. Here's the patch.

Previously, LWLockAcquireWithVar set the variable associated with the
lock atomically with acquiring it. Before the lwlock-scalability
changes, that was straightforward because you held the spinlock anyway,
but it's a lot harder/expensive now. So I changed the way acquiring a
lock with a variable works. There is now a separate flag,
LW_FLAG_VAR_SET, which indicates that the current lock holder has
updated the variable. The LWLockAcquireWithVar function is gone - you
now just use LWLockAcquire(), which always clears the LW_FLAG_VAR_SET
flag, and you can call LWLockUpdateVar() after that if you want to set
the variable immediately. LWLockWaitForVar() always waits if the flag is
not set, i.e. it will not return regardless of the variable's value, if
the current lock-holder has not updated it yet.

This passes make check, but I haven't done any testing beyond that. Does
this look sane to you?

- Heikki