linux-kernel - Re: [PULL REQUEST] i2c-for-6.7-rc2

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Date:   Mon, 20 Nov 2023 09:32:11 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will@...nel.org>
Cc:     Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Rosin <peda@...ntia.se>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Andi Shyti <andi.shyti@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PULL REQUEST] i2c-for-6.7-rc2

On Mon, 20 Nov 2023 at 07:05, Will Deacon <will@...nel.org> wrote:
>
> and I think the high-level problem was something like:
>
> 1. CPU x writes some stuff to memory (I think one example was i2c_dw_xfer()
>    setting 'dev->msg_read_idx' to 0)
> 2. CPU x writes to an I/O register on this I2C controller which generates
>    an IRQ (end of i2c_dw_xfer_init())
> 3. CPU y takes the IRQ
> 4. CPU y reads 'dev->msg_read_idx' and doesn't see the write from (1)
>
> (i2c folks: please chime in if I got this wrong)
>
> the issue being that the writes in (1) are not ordered before the I/O
> access in (2) if the relaxed accessor is used.

Ok, then removing relaxed is indeed the right thing to do. Because
yes, it's an actual ordering issue with the IO write, not some locking
issue.

Thanks for filling in the details, that patch looked iffy to me, but
it does sound like everything is good.

             Linus