linux-kernel - Re: [PATCH] ARC: remove redundant READ

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Message-ID: <e86916c4-1a5c-f89d-bcec-c4ed275aa4be@kernel.org>
Date:   Mon, 18 Apr 2022 12:19:53 -0700
From:   Vineet Gupta <vgupta@...nel.org>
To:     Bang Li <libang.linuxer@...il.com>, vgupta@...nel.org
Cc:     peterz@...radead.org, mingo@...nel.org, valentin.schneider@....com,
        linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARC: remove redundant READ_ONCE() in cmpxchg loop



On 3/18/22 19:03, Bang Li wrote:
> This patch reverts commit 7082a29c22ac ("ARC: use ACCESS_ONCE in cmpxchg
> loop").
>
> It is not necessary to use READ_ONCE() because cmpxchg contains barrier. We
> can get it from commit d57f727264f1 ("ARC: add compiler barrier to LLSC
> based cmpxchg").
>
> Signed-off-by: Bang Li <libang.linuxer@...il.com>

Indeed this is not needed. However have you checked the code with and/wo 
this change to see if it builds the same.

-Vineet

> ---
>   arch/arc/kernel/smp.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c
> index 78e6d069b1c1..56c23f3e1309 100644
> --- a/arch/arc/kernel/smp.c
> +++ b/arch/arc/kernel/smp.c
> @@ -274,7 +274,7 @@ static void ipi_send_msg_one(int cpu, enum ipi_msg_type msg)
>   	 * and read back old value
>   	 */
>   	do {
> -		new = old = READ_ONCE(*ipi_data_ptr);
> +		new = old = *ipi_data_ptr;
>   		new |= 1U << msg;
>   	} while (cmpxchg(ipi_data_ptr, old, new) != old);
>