linux-kernel - Re:Re: [PATCH] kernel/module: Use BUG_ON instead of if condition followed by BUG.

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Message-ID: <AFgAhgByDjWe0vX7lAC2-Ko-.3.1618298474446.Hmail.zhouchuangao@vivo.com>
Date:   Tue, 13 Apr 2021 15:21:14 +0800 (GMT+08:00)
From:   周传高 <zhouchuangao@...o.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re:Re: [PATCH] kernel/module: Use BUG_ON instead of if condition followed by BUG.

>+++ zhouchuangao [30/03/21 05:07 -0700]:
>>It can be optimized at compile time.
>>
>>Signed-off-by: zhouchuangao <zhouchuangao@...o.com>
>
>Hi,
>
>Could you please provide a more descriptive changelog? I.e., Is this
>a fix for a cocinelle warning? What are the optimization(s)?
>
>Thanks,
>
First, 
This patch comes from cocinelle warning.

Second,
#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.

Thanks,
zhouchuangao

>Jessica
>
>>---
>> kernel/module.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>>diff --git a/kernel/module.c b/kernel/module.c
>>index 3047935..f46fc4f 100644
>>--- a/kernel/module.c
>>+++ b/kernel/module.c
>>@@ -1014,8 +1014,8 @@ void __symbol_put(const char *symbol)
>> 	};
>>
>> 	preempt_disable();
>>-	if (!find_symbol(&fsa))
>>-		BUG();
>>+	BUG_ON(!find_symbol(&fsa));
>>+
>> 	module_put(fsa.owner);
>> 	preempt_enable();
>> }
>>-- 
>>2.7.4
>>