Re: Escaping from blocked send() reprised.

On 09/04/2014 04:37 PM, Robert Haas wrote:
> Hrm. So we'd have to block SIGUSR1, check the flag, then use
> pselect() to temporarily unblock SIGUSR1 and wait, then on return
> again unblock SIGUSR1? Doesn't seem very appealing. I think changing
> the signal mask is fast on Linux, but quite slow on at least some
> other UNIX-like platforms. And I've heard that pselect() isn't always
> truly atomic, so we might run into platform-specific bugs, too. I
> wonder if there's a better way e.g. using memory barriers.
>
> WaitLatch: check is_set. if yes then done. otherwise, set signal_me.
> memory barrier. recheck is_set. if not set then wait using
> poll/select. memory barrier. clear signal_me.
> SetLatch: check is_set. if yes then done. otherwise, set is_set.
> memory barrier. check signal_me. if set, then send SIGUSR1.

Doesn't work. No matter what you do, the process running WaitLatch might
receive the signal immediately before it calls poll/select. The signal
handler will run, and the poll/select call will then go to sleep. There
is no way to do this without support from the kernel, that is why
ppoll/pselect exist.

- Heikki