linux-kernel - Re: [RESEND] x86: Refactor __cmpxchg to cmpxchg in lock

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Date:   Tue, 24 Sep 2019 19:47:13 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Philipp K <philipp97kl@...il.com>
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND] x86: Refactor __cmpxchg to cmpxchg in lock_cmos

On Sat, Jan 05, 2019 at 04:29:13PM +0100, Philipp K wrote:
> It is unusual to reference __cmpxchg() from other files than cmpxchg.h and
> similar.
> Instead, cmpxchg() is used, which expands to __cmpxchg() and derives the
> 'size' parameter automatically with sizeof(*(ptr)).
> 
> So clean up the lock_cmos() function by using cmpxchg(), without changing
> the generated code.
> 
> Signed-off-by: Philipp Klocke <philipp97kl@...il.com>
> ---
> 
> This patch was acked by Ingo, so I would expect it to be added to pit.

You mean tip. :)

>  arch/x86/include/asm/mc146818rtc.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/mc146818rtc.h
> b/arch/x86/include/asm/mc146818rtc.h
> index 97198001e567..b72e3bbba0a2 100644
> --- a/arch/x86/include/asm/mc146818rtc.h
> +++ b/arch/x86/include/asm/mc146818rtc.h
> @@ -47,7 +47,7 @@ static inline void lock_cmos(unsigned char reg)
>   cpu_relax();
>   continue;
>   }
> - if (__cmpxchg(&cmos_lock, 0, new, sizeof(cmos_lock)) == 0)
> + if (cmpxchg(&cmos_lock, 0, new) == 0)
>   return;
>   }
>  }
> -- 

I don't know how you created this diff but yours breaks the indentation.
It should look like this:

--
diff --git a/arch/x86/include/asm/mc146818rtc.h b/arch/x86/include/asm/mc146818rtc.h
index 97198001e567..b72e3bbba0a2 100644
--- a/arch/x86/include/asm/mc146818rtc.h
+++ b/arch/x86/include/asm/mc146818rtc.h
@@ -47,7 +47,7 @@ static inline void lock_cmos(unsigned char reg)
 			cpu_relax();
 			continue;
 		}
-		if (__cmpxchg(&cmos_lock, 0, new, sizeof(cmos_lock)) == 0)
+		if (cmpxchg(&cmos_lock, 0, new) == 0)
 			return;
 	}
 }
--

Also, you can simplify it even more by changing it to:

		if (!cmpxchg(&cmos_lock, 0, new))
			return;

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette