This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
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 23:56:00 +0200
- Subject: Re: Async-signal-safe access to __thread variables from dlopen()ed libraries?
- Authentication-results: sourceware.org; auth=none
- References: <CALoOobPJ7G7ciRfc2JwzHjsDTg4-_h-SXqeU1zR4WEzoyQhyNg at mail dot gmail dot com> <523BD470 dot 6090203 at redhat dot com> <CAKOQZ8y85QBkd97cEEmP-4OgE2KizCqknrVR_n44pwBGMs5uAw at mail dot gmail dot com> <523C88D1 dot 6090304 at redhat dot com> <20130920175246 dot GE20515 at brightrain dot aerifal dot cx> <1380705404 dot 8757 dot 1847 dot camel at triegel dot csb> <20131002205046 dot GT20515 at brightrain dot aerifal dot cx> <1381010778 dot 8757 dot 3460 dot camel at triegel dot csb> <20131006053800 dot GQ20515 at brightrain dot aerifal dot cx> <1381095401 dot 8757 dot 3814 dot camel at triegel dot csb> <20131006214057 dot GA14037 at brightrain dot aerifal dot cx>
On Sun, 2013-10-06 at 17:40 -0400, Rich Felker wrote:
> On Sun, Oct 06, 2013 at 11:36:41PM +0200, Torvald Riegel wrote:
> > > My point is that char seems to automatically have this property on any
> > > sane hardware, because, per the the POSIX and C11 memory models, you
> > > can't access char objects as a read-modify-write sequence on a larger
> > > unit of storage; you must perform single-byte accesses.
> >
> > The compiler could still do "arbitrary" stuff to non-atomic and
> > non-volatile char variables (reordering accesses, ...), provided that
> > when assuming a sequential program, the program would behave as if the
> > abstract machine would execute it. The atomics tell the compiler to not
> > assume that this is sequential code; therefore, a char-typed variable
> > doesn't have the stronger properties automatically.
>
> Adding volatile (which, BTW, also needs to be added to sig_atomic_t)
> would avoid these ordering issues.
It would prevent some of them -- but then we're talking not about plain
char-typed variables anymore. Also, volatiles and atomics aren't the
same thing;
> > On the C11 and C++11 models, I think a compiler could also use
> > multi-byte accesses with atomic read-modify-write ops as long as it
> > makes sure that those don't overlap with volatiles or similar.
>
> Is that observably different from a single-byte write? I don't think
> so.
That's the point: The compiler must not necessarily use single-byte
accesses, as you seemed to say (see above).