Re: dynamic shared memory and locks

On Mon, Jan 6, 2014 at 3:57 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Mon, Jan 6, 2014 at 3:32 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> I agree it'd be nicer if we had some better way than mere manual
>>> inspection to enforce proper use of spinlocks; but this change doesn't
>>> seem to me to move the ball downfield by any meaningful distance.
>
>> Well, my thought was mainly that, while it may be a bad idea to take
>> another spinlock while holding a spinlock under any circumstances,
>> somebody might do it and it might appear to work just fine.  The most
>> likely sequences seems to me to be something like SpinLockAcquire(...)
>> followed by LWLockConditionalAcquire(), thinking that things are OK
>> because the lock acquisition is conditional - but in fact the
>> conditional acquire still takes the spinlock unconditionally.
>
> The point I'm making is that no such code should get past review,
> whether it's got an obvious performance problem or not.

Sure, I agree, but we all make mistakes.  It's just a judgement call
as to how likely you think it is that someone might make this
particular mistake, a topic upon which opinions may vary.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company