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Subject: Re: [PATCH] i2c: iproc: fix race between client unreg and isr
From:   Ray Jui <ray.jui@broadcom.com>
To:     Wolfram Sang <wsa@kernel.org>
Cc:     Dhananjay Phadke <dphadke@linux.microsoft.com>,
        Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>,
        linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ray Jui <rjui@broadcom.com>,
        bcm-kernel-feedback-list@broadcom.com
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Date:   Wed, 22 Jul 2020 09:55:17 -0700
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On 7/22/2020 8:51 AM, Ray Jui 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

Actually, thinking about it more, 1. and 2. here need to be an atomic
operation so it needs to be wrapped by a spin lock/unlock (and it is
safe to do so here before calling synchronize_irq below).

Same applies to the two read-modify-write sequneces to enable some of
the slave interrupts in the 'bcm_iproc_i2c_slave_isr' routine.

> 
> 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);
>>>>