2022-03-19 23:17:04

by Bang Li

Subject: [PATCH] ARC: remove redundant READ_ONCE() in cmpxchg loop

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 <[email protected]>
---
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);

--
2.25.1

2022-04-18 22:57:00

by Vineet Gupta

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 <[email protected]>

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);
>