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Date: Sat, 19 Aug 2017 01:01:10 -0700
From: Noah Misch <noah AT leadboat DOT com>
To: Houder <houder AT xs4all DOT nl>
Cc: cygwin AT cygwin DOT com
Subject: Re: Signal delivered while blocked
Message-ID: <20170819080110.GA4022431@rfd.leadboat.com>
References: <20170804074445 DOT GB3154757 AT rfd DOT leadboat DOT com> <8f7f7f8ae098c1321f608645c750cae4 AT smtp-cloud7 DOT xs4all DOT net>
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On Mon, Aug 14, 2017 at 08:03:07AM +0200, Houder wrote:
> On Fri, 4 Aug 2017 00:44:45, 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.

> I do not think that Cygwin is the problem here; your code is the problem
> here, I believe.
> 
> Please, study, for example, chapters 20 and 21 of LPI (Linux Programming
> Interface by Michael Kerrisk).
> 
> (20.10 The Signal Mask (Blocking Signal Delivery)
> (20.13 Changing Signal Dispositions: sigaction())
> 
> You cannot use sigprocmask() like you do; you cannot use SIG_SETMASK as
> a parameter in sigprocmask() within the context of a handler.

What words in those chapters prompted your conclusion?  I see nothing in 20.10
or 20.13 about contextual restrictions on SIG_SETMASK.  Posix mentions no such
restrictions in its sigprocmask() page, and Posix does say:

  The following table defines a set of functions that shall be
  async-signal-safe. Therefore, applications can call them, without restriction,
  from signal-catching functions.
  ...
  sigprocmask()
  -- http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

> Cygwin exhibits misbehaviour in case of your code, because it is slower
> than Linux; however, the code is also wrong for Linux.
> 
> The misbehaviour occurs as result of nested interrupts in case of your
> code (yes, nested interrupts are possible with Linux/Unix!).
> However your code does not experience nesting under Linux, because, as
> I said, Linux is faster than Cygwin.

My code *does* experience signal handler nesting on Linux.  In fact, handlers
nest far more quickly than they do under Cygwin.  However, all its nesting
under Linux takes place outside the sigprocmask()-bounded critical section.
The algorithm that inspired this test case tolerates that nesting, but it does
not tolerate nesting within the critical section.

> The simplest way to exclude one signal from another, is to specify the
> signal (or signals) in the sa_mask of the sigaction parameter ...

That is true.  However, as you discovered in your other thread, it is not an
effective workaround for $SUBJECT.

nm

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