Re: Interruptible sleeps (was Re: CommitFest 2009-07: Yay, Kevin! Thanks, reviewers!)

Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> On 03/09/10 21:16, Tom Lane wrote:
>> It's probably not too unreasonable to assume that pid_t assignment is
>> atomic. But I'm still thinking that we have bigger problems than that
>> if there are really cases where SetLatch can execute at approximately
>> the same time as a latch owner is coming or going.

> I don't see how to avoid it. A walsender, or any process really, can
> exit at any time. It can make the latch inaccessible to others before it
> exits to minimize the window, but it's always going to be possible that
> another process is just about to call SetLatch when you exit.

Well, in that case what we need to do is presume that the latch object
has a continuing existence but the owner/receiver can come and go.
I would suggest that InitLatch needs to initialize the object into a
valid but unowned state; there is *no* deinitialize operation; and
there are AcquireLatch and ReleaseLatch operations to become owner
or stop being owner. We also need to define the semantics of SetLatch
on an unowned latch --- does this set a signal condition that will be
available to the next owner?

This amount of complexity might be overkill for local latches, but I
think we need it for shared ones.

regards, tom lane