Fix signal handler

Marc Lehmann schmorp at schmorp.de
Mon Sep 24 00:27:08 CEST 2012


On Sun, Sep 23, 2012 at 02:43:18PM -0700, Colin McCabe <cmccabe at alumni.cmu.edu> wrote:
> If you check the header file, you find that ev_feed_signal_event is
> writing to a variable of type EV_ATOMIC_T, which turns out to be a
> typedef for sig_atomic_t.

Actually, "volatile sig_atomic_t", unless overriden.

The variable likely in question here is, however, an "ev_loop *" - access
to these is sufficiently atomic on all platforms we plan to support libev
on currently. libev does use memory barriers indeed, but the current
release lacks a few - the CVS version should have those fixes.

> I think memory barriers have been added to C++11, but they haven't
> gotten around to formally adding them to C yet.

Actually they are, kind of, in c11. The trouble with them is that they
only work for atomics, so are not really classical memory barriers.
libecb's code to use these barriers is disabled for that reason.

Or maybe I still don't understand them - fences were apparently added
very late in the process and don't seem to be as well thought-out as the
atomics.

> The reality is, volatile is stupid, and most advanced C users use
> memory barriers instead.  Linus and some other kernel hackers wrote a
> document explaining why:
> http://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt

I think volatiles work very well for the case they were defined for. I
think the reason many people are so pissed at volatile is that they tried
to use it for e.g. threads and after being bitten realised it is indeed
dangerously useless :)

(Notwithstanding, volatile works for threads in many compilers, for
example, microsoft visual c adds memory barriers to volatile accesses, as
an extension - maybe this adds to the problem).

However, for pre-c11 implementations, threads were of no consequence, and
then volatile works just as advertised in a signal handler.

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