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Re: Consensus: data-race freedom as default for glibc code
- From: Torvald Riegel <triegel at redhat dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: "Carlos O'Donell" <carlos at redhat dot com>, Roland McGrath <roland at hack dot frob dot com>, "Joseph S. Myers" <joseph at codesourcery dot com>, GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Mon, 24 Nov 2014 18:38:10 +0100
- Subject: Re: Consensus: data-race freedom as default for glibc code
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On Mon, 2014-11-24 at 17:23 +0100, Florian Weimer wrote:
> On 11/24/2014 04:45 PM, Torvald Riegel wrote:
> > On Mon, 2014-11-24 at 16:25 +0100, Florian Weimer wrote:
> >> On 11/24/2014 04:23 PM, Torvald Riegel wrote:
> >>>> > * Parallel algorithms implemented in glibc itself will be free from
> >>>> > data races (as defined by C11 and its memory model) by default.
> >>> I changed it to:
> >>> * Concurrent code in glibc is free from data races (as defined by C11
> >>> and its memory model) by default.
> >> Fine with me as well (although I think technically, this is about
> >> parallelism, not concurrency :-).
> > And I disagree, that's why I changed it :)
> > We don't parallelize anything in glibc.
> Ugh, we parallelize the execution of threads (unlike the old FSU threads
What I meant was that there's no glibc function that actually splits
work into parallel tasks or such. Yes we run threads concurrently, but
there's no parallel loop, fork/join parallelism, or similar in glibc.
Vector libm functions are parallelism, though.
> > How would you define both categories?
> Concurrency is non-deterministic execution of (sequential) subprograms
> in response to external events. Parallelism is the simultaneous,
> interleaved execution of subprograms. Or something like thatâbasically,
> you can only have high-level race conditions in a concurrent
C11 and C++11 use it differently. For example, C11 (N1570) 126.96.36.199p3:
"Concurrent access to the variable being initialized, even via an atomic
operation, constitutes a data race."
You could also have a look at the shared-memory synchronization
literature to see how they use the terms.
> for low-level things (covered by the C11 memory model), you
> need parallelism.
> But I suspect this particular sub-discussion will never have a fruitful
Maybe. For glibc, I think it's best to use the terminology the C