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From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
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Subject: Re: [PATCH 00/33] Compile-time stack metadata validation
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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.