linux-kernel - Re: [PATCH v2 12/17] md/raid5-cache: Move struct r5l

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Date:   Mon, 30 May 2022 09:48:30 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v2 12/17] md/raid5-cache: Move struct r5l_log definition
 to raid5-log.h



On 2022-05-29 23:59, Christoph Hellwig wrote:
> On Thu, May 26, 2022 at 10:35:59AM -0600, Logan Gunthorpe wrote:
>> Move struct r5l_log definition to raid5-log.h. While this reduces
>> encapsulation, it is necessary for the definition of r5l_log to be
>> public so that rcu_access_pointer() can be used on conf-log in the
>> next patch.
>>
>> rcu_access_pointer(p) doesn't technically dereference the log pointer
>> however, it does use typeof(*p) and some older GCC versions (anything
>> earlier than gcc-10) will wrongly try to dereference the structure:
>>
>>     include/linux/rcupdate.h:384:9: error: dereferencing pointer to
>>                  incomplete type ‘struct r5l_log’
>>
>>       typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
>>            ^
>>
>>     include/linux/rcupdate.h:495:31: note: in expansion of
>>                   macro ‘__rcu_access_pointer’
>>
>>        #define rcu_access_pointer(p) __rcu_access_pointer((p),
>>        __UNIQUE_ID(rcu), __rcu)
>>
>> To prevent this, simply provide the definition where
>> rcu_access_pointer() may be used.
> 
> What about just moving any code that does the rcu_access_pointer on
> conf->log to raid5-cache.c and doing an out of line call for it
> instead?

I guess we could do that. All the inline functions in raid5-log.h are
there to choose between the r5l or the ppl implementaiton. So it that
would mean the r5l implementation would probably be inlined and ppl
would be doing a second out of line call. Not sure if that matters, but
it seems a little odd.

Logan