linux-kernel - Re: [PATCH v6 2/5] i2c: core: run atomic i2c xfer when !preemptible

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Date:   Wed, 14 Jun 2023 19:05:34 -0500
From:   Nishanth Menon <nm@...com>
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>,
        <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 v6 2/5] i2c: core: run atomic i2c xfer when !preemptible

On 21:03-20230509, 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>
> Acked-by: Wolfram Sang <wsa@...nel.org>
> Signed-off-by: Benjamin Bara <benjamin.bara@...data.com>
> ---
>  drivers/i2c/i2c-core.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/i2c-core.h b/drivers/i2c/i2c-core.h
> index 1247e6e6e975..05b8b8dfa9bd 100644
> --- a/drivers/i2c/i2c-core.h
> +++ b/drivers/i2c/i2c-core.h
> @@ -29,7 +29,7 @@ int i2c_dev_irq_from_resources(const struct resource *resources,
>   */
>  static inline bool i2c_in_atomic_xfer_mode(void)
>  {
> -	return system_state > SYSTEM_RUNNING && irqs_disabled();
> +	return system_state > SYSTEM_RUNNING && !preemptible();
>  }
>  
>  static inline int __i2c_lock_bus_helper(struct i2c_adapter *adap)
> 
> -- 
> 2.34.1
> 
Tested-by: Nishanth Menon <nm@...com>

This in addition to a deeper bug in our driver seems to have helped
resolve a report we had been looking at. Tested on beagleplay platform

https://lore.kernel.org/all/ZGeHMjlnob2GFyHF@francesco-nb.int.toradex.com/

-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D