Re: Escaping from blocked send() reprised.

On Thu, Sep 4, 2014 at 9:53 AM, Heikki Linnakangas
<hlinnakangas@vmware.com> wrote:
> 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.

Eesh, I was confused there: ignore me.  I was trying to optimize away
the signal handling but assuming we still had the self-pipe byte.  But
of course in that case we don't need to change anything at all.

I'm going to go get some more caffeine.

-- 
Robert Haas
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