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

[Mike Frysinger <vapier at gentoo dot org>]

On 01 Jun 2015 09:55, Martin Sebor wrote:
> On 06/01/2015 02:38 AM, Florian Weimer wrote:
> > On 05/31/2015 11:37 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).
> >
> > What do you propose as the commit message?
> 
> I usually repeat the ChangeLog entry in my commit messages
> (included in the patch) but I'd be happy to add more detail
> or change the format if there's a preference one way or the
> other.

it would be best if your commit message was useful.  usually that means your 
e-mail that was sent to the list w/the patch explaining what it was all about.
if you're familiar with the linux kernel git message style, then that ;).

> > It's not immediately obvious to me why this change works. :)
> 
> The pthread_cleanup_push and pthread_cleanup_pop macros are
> defined in sysdeps/nptl/pthread.h. In C code outside of
> glibc compiled with -fexceptions (with __EXCEPTIONS defined),
> they make use of GCC attribute cleanup to invoke the cleanup
> handler during stack unwinding. (In C++ they make use of
> a destructor of a class object.) These macros do the right
> thing when an exception is thrown in the code they surround.
> 
> The problem is that the nptl/pthreadP.h header redefines
> the macros for internal use by glibc without the use of
> the cleanup attribute. As a result, when an exception is
> thrown, the cleanup handler is not invoked. This is what
> happens in pthread_once.c.
> 
> By removing the macro redefinitions from nptl/pthreadP.h
> the change causes pthread_once.c to be compiled with the
> more robust macros defined in pthread.h and allows cleanup
> to take place even after an exception has been thrown so
> long as glibc has been compiled with -fexceptions.

they're in there to avoid the PLT right ?  so deleting them means the relocs 
come back.  can't we get both here ?
-mike

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