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

[Yann Sionneau]

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