linux-kernel - Re: [PATCH] i2c: iproc: fix race between client unreg and isr

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Date:   Wed, 22 Jul 2020 17:58:43 -0700
From:   dphadke@...ux.microsoft.com (Dhananjay Phadke)
To:     wsa@...nel.org, ray.jui@...adcom.com
Cc:     rjui@...adcom.com, rayagonda.kokatanur@...adcom.com,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        dphadke@...ux.microsoft.com, bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH] i2c: iproc: fix race between client unreg and isr

Ray Jui <ray.jui@...adcom.com> wrote:

>
> On 7/22/2020 3:41 AM, Wolfram Sang wrote:
> > 
> >>> +	synchronize_irq(iproc_i2c->irq);
> >>
> >> If one takes a look at the I2C slave ISR routine, there are places where
> >> IRQ can be re-enabled in the ISR itself. What happens after we mask all
> >> slave interrupt and when 'synchronize_irq' is called, which I suppose is
> >> meant to wait for inflight interrupt to finish where there's a chance
> >> the interrupt can be re-enable again? How is one supposed to deal with that?
> > 
> > I encountered the same problem with the i2c-rcar driver before I left
> > for my holidays.
> > 
>
> I think the following sequence needs to be implemented to make this
> safe, i.e., after 'synchronize_irq', no further slave interrupt will be
> fired.
>
> In 'bcm_iproc_i2c_unreg_slave':
>
> 1. Set an atomic variable 'unreg_slave' (I'm bad in names so please come
> up with a better name than this)
>
> 2. Disable all slave interrupts
>
> 3. synchronize_irq
>
> 4. Set slave to NULL
>
> 5. Erase slave addresses
>
> In the ISR routine, it should always check against 'unreg_slave' before
> enabling any slave interrupt. If 'unreg_slave' is set, no slave
> interrupt should be re-enabled from within the ISR.
>
> I think the above sequence can ensure no further slave interrupt after
> 'synchronize_irq'. I suggested using an atomic variable instead of
> variable + spinlock due to the way how sync irq works, i.e., "If you use
> this function while holding a resource the IRQ handler may need you will
> deadlock.".
>
> Thanks,
>
> Ray
>
> >>> +	iproc_i2c->slave = NULL;
> >>> +
> >>>  	/* Erase the slave address programmed */
> >>>  	tmp = iproc_i2c_rd_reg(iproc_i2c, S_CFG_SMBUS_ADDR_OFFSET);
> >>>  	tmp &= ~BIT(S_CFG_EN_NIC_SMB_ADDR3_SHIFT);
> >>>