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

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Message-ID: <YpRdL+2e7gngOYPa@infradead.org>
Date:   Sun, 29 May 2022 22:59:11 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        Song Liu <song@...nel.org>,
        Christoph Hellwig <hch@...radead.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 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?