linux-kernel - Re: [PATCH 4/5] irqchip/timer-clint: Use wmb() to order normal writes and IPI writes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAAhSdy1=m+G8qOjGPjKXfw+4t+KFf-4zjBzZfminz8U8=2_-ww@mail.gmail.com>
Date: Tue, 14 Jan 2025 10:04:09 +0530
From: Anup Patel <anup@...infault.org>
To: Xu Lu <luxu.kernel@...edance.com>
Cc: daniel.lezcano@...aro.org, tglx@...utronix.de, paul.walmsley@...ive.com, 
	palmer@...belt.com, lihangjing@...edance.com, xieyongji@...edance.com, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] irqchip/timer-clint: Use wmb() to order normal writes
 and IPI writes

On Mon, Jan 13, 2025 at 8:39 PM Xu Lu <luxu.kernel@...edance.com> wrote:
>
> During an IPI procedure, we need to ensure all previous write operations
> are visible to other CPUs before sending a real IPI. We use wmb() barrier
> to ensure this as CLINT issues IPI via mmio writes.
>
> Signed-off-by: Xu Lu <luxu.kernel@...edance.com>
> ---
>  drivers/clocksource/timer-clint.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/clocksource/timer-clint.c b/drivers/clocksource/timer-clint.c
> index 0bdd9d7ec545..8d73b45f9966 100644
> --- a/drivers/clocksource/timer-clint.c
> +++ b/drivers/clocksource/timer-clint.c
> @@ -48,6 +48,12 @@ EXPORT_SYMBOL(clint_time_val);
>  #ifdef CONFIG_SMP
>  static void clint_send_ipi(unsigned int cpu)
>  {
> +       /*
> +        * Ensure that stores to normal memory are visible to the other CPUs
> +        * before issuing IPI.
> +        */
> +       wmb();
> +

Same comment as PATCH3.

The clint_send_ipi() is called through ipi_mux_send_mask()
which does smp_mb__after_atomic() before calling so no need
for any barrier here. Also, barriers need to be in-pair so adding
a single barrier at random location is inappropriate.
(Refer, https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/irq/ipi-mux.c?h=v6.13-rc7#n78)

Based on the above, this patch is not needed.

Regards,
Anup