[lttng-dev] current_thread_info() not respecting program order with gcc 4.8.x

[Luis Lozano]

Thanks for putting my e-mail in the right thread Alexander.

I also wanted to point out that we recently were hit by the following bug:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58854

which can mess up the handling of SP and cause strange failures in the kernel.

>From the description of the bug in this thread, we don't think the
bugs are related but whoever is going to try removing the "const
attribute" please also make sure that the bug still reproduces after
applying the fix for the GCC bug above.

Luis

On Thu, Nov 21, 2013 at 8:02 AM, Alexander Holler <holler at ahsoftware.de> wrote:
> Am 20.11.2013 01:41, schrieb Linus Torvalds:
>
>> It seems that some gcc alias analysis completely incorrectly thinks
>> that they are not the same memory location, and do not alias. My guess
>> would be that gcc sees that that they are based on the stack pointer
>> with "different" offsets, and decides that the memory locations must
>> be different - without noticing that the "& ~(THREAD_SIZE - 1)" will
>> end up generating the same address for both of them.
>
>
> Luis Lozano just noted (see https://lkml.org/lkml/2013/11/20/625) that
> current_thread_info() has the prototype
>
>
> static inline struct thread_info *current_thread_info(void)
> __attribute_const__;
>
> on arm (and arm64 and unicore32, something the paste from Mathieu missed so
> most people here might have missed that detail too). It's a very good
> finding from Luis.
>
> I'm writing this message because his mail doesn't have an in-reply-to
> header, so it might be missed in this thread.
>
> As Luis said, declaring current_thread_info() as a const function is wrong.
> The gcc manual says:
>
> ----
> const
>
> Many functions do not examine any values except their arguments, and have no
> effects except the return value. Basically this is just slightly more strict
> class than the pure attribute below, since function is not allowed to read
> global memory.
>
> Note that a function that has pointer arguments and examines the data
> pointed to must not be declared const. Likewise, a function that calls a
> non-const function usually must not be const. It does not make sense for a
> const function to return void.
> ----
>
> So current_thread_info() clearly violates the constrain to not read global
> memory. Or in other words, that __attribute_const__ tells gcc explicitly
> that the two reads are pointing to different locations because they are
> assumed to be local (through the const).
>
> So This might be the reason why gcc misses that different calls to
> current_thread_info() might point to the same memory location.
>
>
> As I've an arm gcc 4.8.1 ready too, I'm joining Luis question where the
> reordering can be found. If someone would point me to the source/object
> where this happens, I could have a look if removing the __attribute_const__
> makes a difference.
>
> Regards,
>
> Alexander Holler