On 9/12/06, Gustavo Carneiro <[EMAIL PROTECTED]> wrote:
> On 9/12/06, Adam Olsen <[EMAIL PROTECTED]> wrote:
> > My previous mention of using a *single* flag may survive corruption
> > simply because we can tolerate false positives. Signal handlers would
> > write 0xFFFFFFFF, the poll loop would check if *any* bit is set. If
> > so, write 0x0, read off the fd, then loop around and check it again.
> > If the start of the read() acts as a write-barrier it SHOULD guarantee
> > we don't miss any positive writes.
>
> Why write 0xFFFFFFFF? Why can't the variable be of a "volatile
> char" type? Assuming sizeof(char) == 1, please don't tell me
> architecture XPTO will write the value 4 bits at a time! :P
Nope. It'll write 32 bits, then break that up into 8 bits :)
Although, at the moment I can't fathom what harm that would cause...
For the record, all volatile does is prevent compiler reordering
across sequence points.
Interestingly, it seems "volatile sig_atomic_t" is the correct way to
declare a variable for (single-threaded) signal handling. Odd that
volatile didn't show up in any of the previous documentation I read..
> I see your point of using a flag to avoid the read() syscall most of
> the time. Slightly more complex, but possibly worth it.
>
> I was going to describe a possible race condition, then wrote the
> code below to help explain it, modified it slightly, and now I think
> the race is gone. In any case, the code might be helpful to check if
> we are in sync. Let me know if you spot any race condition I missed.
>
>
> static volatile char signal_flag;
> static int signal_pipe_r, signal_pipe_w;
>
> PyErr_CheckSignals()
> {
> if (signal_flag) {
> char signum;
> signal_flag = 0;
> while (read(signal_pipe_r, &signum, 1) == 1)
> process_signal(signum);
> }
> }
I'd prefer this to be a "while (signal_flag)" instead, although it
should technically work either way.
> static void
> signal_handler(int signum)
> {
> char signum_c = signum;
> signal_flag = 1;
> write(signal_pipe_w, &signum_c, 1);
> }
This is wrong. PyErr_CheckSignals could check and clear signal_flag
before you reach the write() call. "signal_flag = 1" should come
after.
--
Adam Olsen, aka Rhamphoryncus
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
http://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
