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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