David Laight <david.lai...@aculab.com> wrote:
> From: Oleg Nesterov
> > Sent: 29 May 2019 17:12
> > Al, Linus, Eric, please help.
> >
> > The previous discussion was very confusing, we simply can not understand
> > each
> > other.
> >
> > To me everything looks very simple and clear, but perhaps I missed something
> > obvious? Please correct me.
> >
> > I think that the following code is correct
> >
> > int interrupted = 0;
> >
> > void sigint_handler(int sig)
> > {
> > interrupted = 1;
> > }
> >
> > int main(void)
> > {
> > sigset_t sigint, empty;
> >
> > sigemptyset(&sigint);
> > sigaddset(&sigint, SIGINT);
> > sigprocmask(SIG_BLOCK, &sigint, NULL);
> >
> > signal(SIGINT, sigint_handler);
> >
> > sigemptyset(&empty); // so pselect() unblocks SIGINT
> >
> > ret = pselect(..., &empty);
> ^^^^^ sigint
> >
> > if (ret >= 0) // sucess or timeout
> > assert(!interrupted);
> >
> > if (interrupted)
> > assert(ret == -EINTR);
> > }
> >
> > IOW, if pselect(sigmask) temporary unblocks SIGINT according to sigmask,
> > this
> > signal should not be delivered if a ready fd was found or timeout. The
> > signal
> > handle should only run if ret == -EINTR.
>
> Personally I think that is wrong.
> Given code like the above that has:
> while (!interrupted) {
> pselect(..., &sigint);
> // process available data
> }
>
> You want the signal handler to be executed even if one of the fds
> always has available data.
> Otherwise you can't interrupt a process that is always busy.
Agreed... I believe cmogstored has always had a bug in the way
it uses epoll_pwait because it failed to check interrupts if:
a) an FD is ready + interrupt
b) epoll_pwait returns 0 on interrupt
The bug remains in userspace for a), which I will fix by adding
an interrupt check when an FD is ready. The window is very
small for a) and difficult to trigger, and also in a rare code
path.
The b) case is the kernel bug introduced in 854a6ed56839a40f
("signal: Add restore_user_sigmask()").
I don't think there's any disagreement that b) is a kernel bug.
So the confusion is for a), and POSIX is not clear w.r.t. how
pselect/poll works when there's both FD readiness and an
interrupt.
Thus I'm inclined to believe *select/*poll/epoll_*wait should
follow POSIX read() semantics:
If a read() is interrupted by a signal before it reads any data, it shall
return −1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully read
some data, it shall return the number of bytes read.
> One option is to return -EINTR if a signal is pending when the mask
> is updated - before even looking at anything else.
>
> Signals that happen later on (eg after a timeout) need not be reported
> (until the next time around the loop).
I'm not sure that's necessary and it would cause delays in
signal handling.
