linux-kernel - Re: [PATCH v4 2/4] i2c: core: run atomic i2c xfer when !preemptible

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Date:   Thu, 13 Apr 2023 21:51:25 +0200
From:   Wolfram Sang <wsa@...nel.org>
To:     Benjamin Bara <bbara93@...il.com>
Cc:     Lee Jones <lee@...nel.org>, rafael.j.wysocki@...el.com,
        dmitry.osipenko@...labora.com, peterz@...radead.org,
        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 v4 2/4] i2c: core: run atomic i2c xfer when !preemptible

On Thu, Apr 13, 2023 at 09:46:40AM +0200, Benjamin Bara wrote:
> From: Benjamin Bara <benjamin.bara@...data.com>
> 
> Since bae1d3a05a8b, i2c transfers are non-atomic if preemption is
> disabled. However, non-atomic i2c transfers require preemption (e.g. in
> wait_for_completion() while waiting for the DMA).
> 
> panic() calls preempt_disable_notrace() before calling
> emergency_restart(). Therefore, if an i2c device is used for the
> restart, the xfer should be atomic. This avoids warnings like:
> 
> [   12.667612] WARNING: CPU: 1 PID: 1 at kernel/rcu/tree_plugin.h:318 rcu_note_context_switch+0x33c/0x6b0
> [   12.676926] Voluntary context switch within RCU read-side critical section!
> ...
> [   12.742376]  schedule_timeout from wait_for_completion_timeout+0x90/0x114
> [   12.749179]  wait_for_completion_timeout from tegra_i2c_wait_completion+0x40/0x70
> ...
> [   12.994527]  atomic_notifier_call_chain from machine_restart+0x34/0x58
> [   13.001050]  machine_restart from panic+0x2a8/0x32c
> 
> Use !preemptible() instead, which is basically the same check as
> pre-v5.2.
> 
> Fixes: bae1d3a05a8b ("i2c: core: remove use of in_atomic()")
> Cc: stable@...r.kernel.org # v5.2+
> Suggested-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> Signed-off-by: Benjamin Bara <benjamin.bara@...data.com>

So, with Peter's input and me checking again:

Acked-by: Wolfram Sang <wsa@...nel.org>

I assume this shall go in via the mfd-tree. Let me know if I should pick
it instead.


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