linux-kernel - Re: [PATCH] irqchip: riscv: Order normal writes and IPI writes

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Message-ID: <87ldv9afso.ffs@tglx>
Date: Fri, 17 Jan 2025 11:35:51 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Charlie Jenkins <charlie@...osinc.com>, Xu Lu <luxu.kernel@...edance.com>
Cc: anup@...infault.org, 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] irqchip: riscv: Order normal writes and IPI writes

Charlie!

On Thu, Jan 16 2025 at 13:09, Charlie Jenkins wrote:
> On Thu, Jan 16, 2025 at 08:07:10PM +0800, Xu Lu wrote:
>> Replace writel_relaxed() with writel() when issuing IPI to ensure all
>> previous write operations made by current CPU are visible to other CPUs.
>
> Did you experience an ordering issue from this?

That's not the right question.

      CPU 0                     CPU 1
      store A   // data
      store B   // IPI
                                IPI handler
                                load A

The real question is whether the RISC-V memory model guarantees under
all circumstances that A is globally visible before the IPI handler
load. If so, then the writel_relaxed() is fine. If not, the fence is
required.

That's not a question of observation. It's a question of facts defined
by the architecture. People have wasted months to analyze such fails
which tend to happen once every blue moon with no other trace than
"something went wrong" ....

> - Charlie

Please trim your replies...

Thanks,

        tglx