Re: [PATCH 00/33] Compile-time stack metadata validation

Peter Zijlstra
2016-02-12T20:11:05Z

On Fri, Feb 12, 2016 at 12:32:06PM -0600, Josh Poimboeuf wrote:
> What I actually see in the listing is:
>
> decl __percpu_prefix:__preempt_count
> je 1f:
> ....
> 1:
> call ___preempt_schedule
>
> So it puts the "call ___preempt_schedule" in the slow path.

Ah yes indeed. Same difference though.

> I also don't see how that would be related to the use of the asm
> statement in the __preempt_schedule() macro. Doesn't the use of
> unlikely() in preempt_enable() put the call in the slow path?

Sadly no, unlikely() and asm_goto don't work well together. But the slow
path or not isn't the reason we do the asm call thing.

> #define preempt_enable() \
> do { \
> barrier(); \
> if (unlikely(preempt_count_dec_and_test())) \
> preempt_schedule(); \
> } while (0)
>
> Also, why is the thunk needed? Any reason why preempt_enable() can't be
> called directly from C?

That would make the call-site save registers and increase the size of
every preempt_enable(). By using the thunk we can do callee saved
registers and avoid blowing up the call site.