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Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
- From: Torvald Riegel <triegel at redhat dot com>
- To: Rich Felker <dalias at aerifal dot cx>
- Cc: "Carlos O'Donell" <carlos at redhat dot com>, Ian Lance Taylor <iant at google dot com>, Paul Pluzhnikov <ppluzhnikov at google dot com>, Roland McGrath <roland at hack dot frob dot com>, Richard Henderson <rth at twiddle dot net>, GNU C Library <libc-alpha at sourceware dot org>, Andrew Hunter <ahh at google dot com>, Alexandre Oliva <aoliva at redhat dot com>
- Date: Sun, 06 Oct 2013 00:06:18 +0200
- Subject: Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
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On Wed, 2013-10-02 at 16:50 -0400, Rich Felker wrote:
> On Wed, Oct 02, 2013 at 11:16:44AM +0200, Torvald Riegel wrote:
> > On Fri, 2013-09-20 at 13:52 -0400, Rich Felker wrote:
> > > On Fri, Sep 20, 2013 at 01:41:37PM -0400, Carlos O'Donell wrote:
> > > > Will our TLS variables become lock-free atomic objects?
> > >
> > > I don't see how this question is related at all. Atomic in the C11
> > > sense has to do with synchronization between processors, not signals.
> > > The memory model for access to objects from signal handlers should not
> > > define the behavior when the signal handler accesses an object whose
> > > modification the signal handler interrupted, except for objects of
> > > type sig_atomic_t or character types (I added the latter because the
> > > C11 memory model already requires byte-granularity write operations),
> >
> > You mean plain character types like a non-atomic char? I doubt that
> > trying to give guarantees for those is right because it would prevent
> > optimizations.
>
> The optimizations are already forbidden by C11 and POSIX memory
> models. For example, if you have:
>
> char x[2];
>
> it's legal for threads A and B to access x[0] and x[1] concurrently
> without any locking.
Those are separate memory locations, and this holds independent of the
type. Thus, I still don't see why you see similarities between a plain
char and, say, sig_atomic_t.