Date:   Wed, 27 Sep 2017 14:06:37 +0200
From:   Kamil Konieczny <k.konieczny@...tner.samsung.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Vladimir Zapolskiy <vz@...ia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] crypto: s5p-sss: Add HASH support for Exynos

On 26.09.2017 21:28, Krzysztof Kozlowski wrote:
> On Tue, Sep 26, 2017 at 05:54:42PM +0200, Kamil Konieczny wrote:
>> On 25.09.2017 20:27, Krzysztof Kozlowski wrote:
>> [...]
>>>>>> +	struct tasklet_struct		hash_tasklet;
>>>>>> +	u8				xmit_buf[BUFLEN] SSS_ALIGNED;
>>>>>> +
>>>>>> +	unsigned long			hash_flags;
>>>>>
>>>>> This should be probably DECLARE_BITMAP.
>>>>
>>>> I do not get it, it is used for bits HASH_FLAGS_... and is not HW related.
>>>> This will grow this patch even longer.
>>>
>>> Why longer? Only declaration changes, rest should be the same.
>>> Just instead of playing with bits over unsigned long explicitly use a
>>> type for that purpose - DECLARE_BITMAP.
>>
>> It will make code ugly, using &dev->hash_flags[0] instead of &dev->hash_flags.
> 
> Wait, all the set_bit and clear_bit should be even simpler:
> 	-set_bit(HASH_FLAGS_DMA_READY, &dev->hash_flags);
> 	+set_bit(HASH_FLAGS_DMA_READY, dev->hash_flags);
> This looks actually better..
> 
>> I have 9 bits used, and I will not anticipate it grow above 64.
> 
> You mean 32.
> 
>> If you insist, I can add DECLARE_BITMAP and rewrite all calls to _bits()
>> functions.
> 
> Actually not, I do not insist. Just for me it makes the code simpler
> (lack of "&" operand) and more robust as you explicitly declare number
> of used bits (instead of assumption that your data type on given
> compiler matches the amount of flags). Both arguments are not that
> important.

I see, but I will postpone this to later refactoring and code cleanup,
as none crypto drivers uses it now, and the declaration after macro unwind
will look like:

unsigned long			hash_flags[1];

[...]
>>>>>> +	} else {
>>>>>> +		ctx->flags |= BIT(HASH_FLAGS_ERROR);
>>>>>> +	}
>>>>>> +
>>>>>> +	/* atomic operation is not needed here */
>>>>>
>>>>> Ok, but why?
>>>>>
>>>>
>>>> Good question, I frankly copy&paste this. The hash vars in s5p_aes_dev
>>>> are no longer used as all transfers ended, we have req context here
>>>> so after complete we can work on next request,
>>>> and clearing bit HASH_FLAGS_BUSY allows this.
>>>
>>> I think you need them. Here and few lines before. In
>>> s5p_hash_handle_queue() you use spin-lock because it might be executed
>>> multiple times. Having that assumption (multiple calls to
>>> s5p_hash_handle_queue()) you can have:
>>>
>>> Thread #1:                     Thread #2
>>>  s5p_hash_finish_req()         s5p_hash_handle_queue()
>>>  dd->hash_flags &= ...
>>>    which equals to:
>>
>> somewhere here will be (in Thread #2):
>> 			if(test_bit(HASH_FLAGS_BUSY,dd->hash_flags)) {
>> 					unlock, return; }
> 
> Right, good point! The HASH_FLAGS_BUSY bit is cleared only in
> s5p_hash_finish_req() which will be executed only if the bit was set
> (checked with test_bits()). This should be then put next to the sentence
> "atomics are not needed here".
> 
>>
>>>    tmp = dd->hash_flags;
>>>    tmp &= ~(BIT...)
>>>                                  set_bit(HASH_FLAGS_BUSY, &dd->hash_flags);
>>>    dd->hash_flags = tmp
>>>
>>> Which means that in last assignment you effectively lost the
>>> HASH_FLAGS_BUSY bit.
>>>
>>> In general, you need to use atomics (or locks) everywhere, unless you
>>> are 100% sure that given lockless section cannot be executed with other
>>> code which uses locks.  Otherwise the atomics/locks become uneffective.
>>
>> I can add spinlock as I am not 100% sure if below is true:
>>
>> HASH_FLAGS_BUSY is protected by order of instructions and by test_bit.
> 
> In SMP code, one should not expect too much of order. :)
> For sections not guarded by barriers (thus also locks) compiler can
> re-order a lot. CPU can re-order even more. Although these
> test_bit() and clear_bit() are atomic operations but they do not provide
> barriers so they can be re-ordered.
> 
>> First, this bit is set only in one place, in s5p_hash_handle_queue() and cleared 
>> also only in one place, s5p_hash_finish_req().
>>
>> In function s5p_hash_handle_queue(), after spinlock, bit HASH_FLAGS_BUSY
>> is tested and if set, there is unlock and exit, and the flow reaches next
>> instructions only when this bit was not set. Setting bit is in the same spinlock
>> section, so as you write, concurrent calls to s5p_hash_handle_queue are
>> protected by spinlock.
> 
> Yes, but on the other hand writing the HASH_FLAGS_BUSY bit in
> s5p_hash_finish_req() is not protected anyhow and it will be called on
> different CPU in different thread (through tasklet). Although
> s5p_hash_finish_req() is called only after checking the bits... but it
> is I think unusual pattern (in s5p_hash_tasklet_cb()):
>  - test_bit(hash_flags)
>  - non-atomic write to hash_flags
>  - test_bit(hash_flags)
> 

OK, as I wrote I am not sure, so I add spinlock in s5p_hash_finish_req() to
protect code clearing HASH_FLAGS_BUSY.

-- 
Best regards,
Kamil Konieczny
Samsung R&D Institute Poland

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