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Re: Signal delivered while blocked




On Fri, Aug 04, 2017 at 11:58:51AM -0700, Kaz Kylheku wrote:
> On 04.08.2017 10:02, Corinna Vinschen wrote:
> >On Aug  4 00:44, Noah Misch wrote:
> >>The attached demonstration program blocks signals (with sigprocmask())
> >>to
> >>achieve mutual exclusion between signal handlers.  It aborts upon
> >>receipt of a
> >>blocked signal.  On "CYGWIN_NT-10.0 2.7.0(0.306/5/3) 2017-02-12 13:18
> >>x86_64",
> >>signals regularly arrive despite being blocked.  Essential parts of the
> >>program include handling two signal numbers and having handlers run for
> >>at
> >>least 1-2ms; this problem goes away if I remove one of those
> >>attributes.
> >>GNU/Linux, AIX, Solaris, and "CYGWIN_NT-6.0 1.7.27(0.271/5/3)
> >>2013-12-09 11:57
> >>i686" never deliver a blocked signal to this program.  I think this
> >>Cygwin
> >>behavior is non-conforming.
> >
> >Thanks for the testcase.  I debugged this a while today but the problem
> >is far from trivial, apparently.  Don't hold your breath for a quick
> >solution.

Understood.  Thanks for studying it.

> The test case depends on accesses to the global variable sigblocked not
> to be reordered w.r.t. siggprocmask calls.
> 
> It is important that the variable not be set to 1 until after the signals
> are
> blocked, and be reset to 0 until after they are unblocked.
> 
> Thus, the variable should be declared volatile.

Agreed, but ...

> Although I would be surprised if this were actually happening

... indeed, that didn't change the result.

> Also, related remarks: for the reason that we can't factor out compiler
> behavior, with absolute certainty, it would be good to mention not only
> the system versions but also GCC. The compiler differs, obviously,
> between Cygwin 1.7 and 2.7; not to mention that the case is reported
> against i686 of the one, and x86_74 of the other.

The Cygwin 2.7 system has gcc-core 5.4.0-1.  The unaffected Cygwin 1.7 system
has gcc-core 4.8.2-2.

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