linux-kernel - Re: [PATCH v3 2/4] i2c: core: run atomic i2c xfer when !preemptible

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]

Date:   Tue, 4 Apr 2023 10:22:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Benjamin Bara <bbara93@...il.com>
Cc:     Wolfram Sang <wsa@...nel.org>, Lee Jones <lee@...nel.org>,
        rafael.j.wysocki@...el.com, dmitry.osipenko@...labora.com,
        jonathanh@...dia.com, richard.leitner@...ux.dev,
        treding@...dia.com, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-tegra@...r.kernel.org,
        Benjamin Bara <benjamin.bara@...data.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 2/4] i2c: core: run atomic i2c xfer when !preemptible

On Sun, Apr 02, 2023 at 12:04:48PM +0200, Benjamin Bara wrote:
> On Wed, 29 Mar 2023 at 21:50, Wolfram Sang <wsa@...nel.org> wrote:
> > Could you make sure please?
> 
> Sure, I'll try. The check before bae1d3a was:
> in_atomic() || irqs_disabled()
> which boils down to:
> (preempt_count() != 0) || irqs_disabled()
> preemptible() is defined as:
> (preempt_count() == 0 && !irqs_disabled())
> 
> so this patch should behave the same as pre-v5.2, but with the
> additional system state check. From my point of view, the additional
> value of the in_atomic() check was that it activated atomic i2c xfers
> when preemption is disabled, like in the case of panic(). So reverting
> that commit would also re-activate atomic i2c transfers during emergency
> restarts. However, I think considering the system state makes sense
> here.
> 
> From my understanding, non-atomic i2c transfers require enabled IRQs,
> but atomic i2c transfers do not have any "requirements". So the
> irqs_disabled() check is not here to ensure that the following atomic
> i2c transfer works correctly, but to use non-atomic i2c xfer as
> long/often as possible.
> 
> Unfortunately, I am not sure yet about !CONFIG_PREEMPTION. I looked into
> some i2c-bus implementations which implement both, atomic and
> non-atomic. As far as I saw, the basic difference is that the non-atomic
> variants usually utilize the DMA and then call a variant of
> wait_for_completion(), like in i2c_imx_dma_write() [1]. However, the
> documentation of wait_for_completion [2] states that:
> "wait_for_completion() and its variants are only safe in process context
> (as they can sleep) but not (...) [if] preemption is disabled".
> Therefore, I am not quite sure yet if !CONFIG_PREEMPTION uses the
> non-atomic variant at all or if this case is handled differently.
> 
> > Asking Peter Zijlstra might be a good idea.
> > He helped me with the current implementation.
> 
> Thanks for the hint! I wrote an extra email to him and added him to CC.

So yeah, can't call schedule() if non preemptible (which is either
preempt_disable(), local_bh_disable() (true for bh handlers) or
local_irq_disable() (true for IRQ handlers) and mostly rcu_read_lock()).

You can mostly forget about CONFIG_PREEMPT=n (or more specifically
CONFIG_PREMPT_COUNT=n) things that work for PREEMPT typically also work
for !PREEMPT.

The question here seems to be if i2c_in_atomic_xfer_mode() should have
an in_atomic() / !preemptible() check, right? IIUC Wolfram doesn't like
it being used outside of extra special cicumstances?