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

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