[PATCH] pthread_once hangs when init routine throws an exception [BZ #18435]

[Martin Sebor]

Attached is an updated version of the patch that addresses
the LDFLAGS -> LDLIBS comment. Retested on ppc64.

Is it okay to commit?

Martin

On 05/31/2015 03:37 PM, Martin Sebor wrote:
> The C++ 2011 std::call_once function is specified to allow
> the initialization routine to exit by throwing an exception.
> Such an execution, termed exceptional, requires call_once to
> propagate the exception to its caller. A program may contain
> any number of exceptional executions but only one returning
> execution (which, if it exists, must be the last execution
> with the same once flag).
>
> On POSIX systems such as Linux std::call_once is implemented
> in terms of pthread_once. However, as discussed in libstdc++
> bug 66146 - "call_once not C++11-compliant on ppc64le," GLIBC's
> pthread_once hangs when the initialization function exits by
> throwing an exception on at least arm and ppc64 (though
> apparently not on x86_64). This effectively prevents call_once
> from conforming to the C++ requirements since there doesn't
> appear to be a thread-safe way to work around this problem in
> libstdc++.
>
> The attached patch changes pthread_once to handle gracefully
> init functions that exit by throwing exceptions. It has been
> tested on ppc64, ppc64le, and x86_64 with no regressions.
>
> During the discussion of the bug concerns were raised about
> whether the use case of throwing exceptions from the
> pthread_once init routine is intended to be supported either
> by POSIX, or by GLIBC. After some research I believe that both
> POSIX and GLIBC have, in fact, intended to support it, for at
> least two reasons:
>
> First, the POSIX Rationale states in section Thread Cancellation
> Overview, under Thread Cancellation Cleanup Handlers, that:
>
>    it is an explicit goal of POSIX.1-2008 to be compatible with
>    existing exception facilities and languages having exceptions.
>
> Second, as is evident from the comment above the pthread_once
> declaration in GLIBC (quoted below), GLIBC too has intended
> to support this use case since 2004 when the comment was
> added (and the __THROW specification removed from the API):
>
>     ...
>     The initialization functions might throw exception which
>     is why this function is not marked with __THROW.  */
>
> Martin

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