2021-03-30 12:00:04

by zhouchuangao

Subject: [PATCH] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.

It can be optimized at compile time.

Signed-off-by: zhouchuangao <[email protected]>
---
arch/arm64/kernel/probes/kprobes.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index 66aac28..ecf0f61 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -264,8 +264,7 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
* normal page fault.
*/
instruction_pointer_set(regs, (unsigned long) cur->addr);
- if (!instruction_pointer(regs))
- BUG();
+ BUG_ON(!instruction_pointer(regs));

if (kcb->kprobe_status == KPROBE_REENTER)
restore_previous_kprobe(kcb);
--
2.7.4

2021-03-30 12:10:17

by Will Deacon

Subject: Re: [PATCH] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.

On Tue, Mar 30, 2021 at 04:57:50AM -0700, zhouchuangao wrote:
> It can be optimized at compile time.

Hmm, I don't see it (and I also don't understand why we care). Do you have
numbers showing that this is worthwhile?

Will

2021-04-01 11:01:04

by zhouchuangao

Subject: Re:Re: [PATCH] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.

>On Tue, Mar 30, 2021 at 04:57:50AM -0700, zhouchuangao wrote:>> It can be optimized at compile time.
>
>Hmm, I don't see it (and I also don't understand why we care). Do you have
>numbers showing that this is worthwhile?
>

#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while (0)

BUG_ON uses unlikely in if(). Through disassembly, we can see that
brk #0x800 is compiled to the end of the function.
As you can see below:
......
ffffff8008660bec: d65f03c0 ret
ffffff8008660bf0: d4210000 brk #0x800

Usually, the condition in if () is not satisfied. For the multi-stage pipeline,
we do not need to perform fetch decode and excute operation on brk
instruction.

In my opinion, this can improve the efficiency of the multi-stage pipeline.

zhouchuangao

>Will

2021-04-01 21:22:33

by Masami Hiramatsu

Subject: Re: [PATCH] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.

On Tue, 30 Mar 2021 04:57:50 -0700
zhouchuangao <[email protected]> wrote:

> It can be optimized at compile time.
>

Anyway, this seems to make the code simpler.

Reviewed-by: Masami Hiramatsu <[email protected]>

Thanks!

> Signed-off-by: zhouchuangao <[email protected]>
> ---
> arch/arm64/kernel/probes/kprobes.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 66aac28..ecf0f61 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -264,8 +264,7 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
> * normal page fault.
> */
> instruction_pointer_set(regs, (unsigned long) cur->addr);
> - if (!instruction_pointer(regs))
> - BUG();
> + BUG_ON(!instruction_pointer(regs));
>
> if (kcb->kprobe_status == KPROBE_REENTER)
> restore_previous_kprobe(kcb);
> --
> 2.7.4
>


--
Masami Hiramatsu <[email protected]>

2021-04-14 22:39:18

by Catalin Marinas

Subject: Re: [PATCH] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.

On Tue, 30 Mar 2021 04:57:50 -0700, zhouchuangao wrote:
> It can be optimized at compile time.

Applied to arm64 (for-next/misc), it saves one line ;). Thanks!

[1/1] arm64/kernel/probes: Use BUG_ON instead of if condition followed by BUG.
https://git.kernel.org/arm64/c/839157876f97

--
Catalin