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

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Message-ID: <YJvP7JLKwJbcEYIg@p200300cbcf361a0029e37a38368d6727.dip0.t-ipconnect.de>
Date:   Wed, 12 May 2021 14:54:04 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     zhouchuangao <zhouchuangao@...o.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/module: Use BUG_ON instead of if condition
 followed by BUG

+++ zhouchuangao [09/05/21 18:38 -0700]:
>This patch comes from cocinelle warning.

Please include the output of the cocinelle warning here as well.

See commit 56149c8cd820 ("media: pci: saa7164-core.c: replace if
(cond) BUG() with BUG_ON()") for an example. Thanks.

>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.
>
>Signed-off-by: zhouchuangao <zhouchuangao@...o.com>
>---
> kernel/module.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
>diff --git a/kernel/module.c b/kernel/module.c
>index b5dd92e..faf9114 100644
>--- a/kernel/module.c
>+++ b/kernel/module.c
>@@ -1014,8 +1014,7 @@ 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
>