[lttng-dev] RFC: Fix crash in dlerror()

[Woegerer, Paul]

On 02/14/2014 11:30 AM, Alexander Monakov wrote:
> On Thu, 13 Feb 2014, Stefan Seefeld wrote:
>> Our compilation unit defines a bunch of functions with external linkage,
>> which access cur_alloc. And since gcc has no way to rule out that the
>> call to dlsym() will not cause any of these functions to be called, it
>> mustn't make any assumptions about whether or not the first
>> initialization of cur_alloc is redundant or not, and thus shouldn't
>> elide it.
> Stefan, LTTng developers,
>
> The problem here is that glibc declares dlsym() with __attribute__((leaf))
> (see the definition of __THROW in /usr/include/sys/cdefs.h and the difference
> from __THROWNL).  Presence of the attribute allows the compiler to assume that
> no functions from the current compilation unit will be called from dlsym, and
> thus there's no need to write back potentially escaping data.

So the CMM_ACCESS_ONCE's I suggested would only be required if dlsym()
would call back into the wrapper from another thread, right ?

Thanks,
Paul

>
> glibc used to have __THROW annotations on dlopen() as well, and they changed
> it to __THROWNL (removing the "leaf" attribute) after it was pointed out that
> dlopen will call constructors:
>
> https://sourceware.org/ml/libc-alpha/2013-08/msg00465.html
>
> Unfortunately when I pointed out that dlsym is not really "leaf" as well, my
> argument was dismissed:
>
> https://sourceware.org/ml/libc-alpha/2013-09/msg00012.html
>
> Please consider filing a glibc bug or otherwise reopening that discussion.
>
> Hope that helps,
> Alexander


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Paul Woegerer, SW Development Engineer
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