linux-kernel - Re: [PATCH] i2c: designware: Fix corrupted memory seen in the ISR

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Message-ID: <038e40c8-d6cd-150e-933e-5b2d2d7af7c5@kalrayinc.com>
Date:   Wed, 13 Sep 2023 13:32:43 +0200
From:   Yann Sionneau <ysionneau@...rayinc.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Jan Bottorff <janb@...amperecomputing.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jan Dabros <jsd@...ihalf.com>,
        Andi Shyti <andi.shyti@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c: designware: Fix corrupted memory seen in the ISR


On 13/09/2023 13:22, Andy Shevchenko wrote:
> On Wed, Sep 13, 2023 at 11:04:00AM +0200, Yann Sionneau wrote:
>> On 13/09/2023 03:03, Jan Bottorff wrote:
> ...
>
>>> +	/*
>>> +	 * To guarantee data written by the current core is visible to
>>> +	 * all cores, a write barrier is required. This needs to be
>>> +	 * before an interrupt causes execution on another core.
>>> +	 * For ARM processors, this needs to be a DSB barrier.
>>> +	 */
>>> +	wmb();
>> Apart from the commit message it looks good to me.
>>
>> If I understand correctly without this wmb() it is possible that the writes
>> to dev->msg_write_idx , dev->msg_read_idx = 0 etc would not yet be visible
>> to another CPU running the ISR handler right after enabling those.
> If this is the case, shouldn't we rather use READ_ONCE()/WRITE_ONCE() where
> appropriate?
>
To my knowledge the READ_ONCE()/WRITE_ONCE() only imply the use of 
volatile to access memory thus preventing the compiler to do weird 
optimizations like merging store/loads, moving store/loads, removing 
them etc

They don't imply a memory barrier.

Some systems need a memory barrier, to emit a "fence" like instruction, 
so that the pipeline stalls waiting for the store to "finish", for 
systems where the writes are posted.

Regards,

-- 

Yann