Re: Latches with weak memory ordering (Re: max_wal_senders must die)

On Friday 19 November 2010 15:38:37 Robert Haas wrote:
> Eh, really? If there's a workaround for platforms for which we don't
> know what the appropriate read-fencing incantation is, then I'd feel
> more comfortable about doing this. But I don't see how to make that
> work. The whole problem here is that API is designed in such a way
> that the signal handler might be invoked when the lock that it needs
> to grab is already held by the same process. The reason memory
> barriers solve the problem is because they'll be atomically released
> when we jump into the signal handler, but that is not true of a
> spin-lock or a semaphore.
Well, its not generally true - you are right there. But there is a wide range
for syscalls available where its inherently true (which is what I sloppily
referred to). And you are allowed to call a, although quite restricted, set of
system calls even in signal handlers. I don't have the list for older posix
versions in mind, but for 2003 you can choose something from several like
write, lseek,setpgid which inherently have to serialize. And I am quite sure
there were sensible calls for earlier versions.

Andres