Re: [PATCH] [AArch64] support -mfentry feature for arm64

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On Mon, Apr 18, 2016 at 02:12:09PM +0200, Michael Matz wrote:
> Hi,
> 
> On Sun, 17 Apr 2016, Alexander Monakov wrote:
> 
> > I've noticed an issue in my (and probably Michael's) solution: if 
> > there's a thread that made it past the first nop, but is still executing 
> > the nop pad, it's unsafe to replace the nops.

Yeah, this issue also trapped me before :)
 
> To solve that, it 
> > suffices to have a forward branch in place of the first nop to begin 
> > with (i.e. have the compiler emit it).
>
> True.  I wonder if the generic solution in GCC should do that always or if 
> the patch infrastructure should do that to enable more freedom like doing 
> this:
> 
> > But if Szabolcs' two-instruction 
> > sequence in the adjacent subthread is sufficient, this is moot.
> 
> .  It can also be solved by having just one NOP after the function label, 
> and a number of them before, then no thread can be in the nop pad.  That 
> seems to indicate that GCC should not try to be too clever and simply 
> leave the specified number of nops before and after the function label, 
> leaving safety measures to the patching infrastructure.

I don't get this idea very well.
How can the instructions *before* a function label be executed
after branching into this function?

Thanks,
-Takahiro AKASHI

> 
> Ciao,
> Michael.

-- 
Thanks,
-Takahiro AKASHI