linux-kernel - Re: [PATCH v1 12/15] md/raid5-cache: Add RCU protection to conf->log accesses

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Message-ID: <f270d92d-7c34-2f58-89c5-ccb7a8a37872@deltatee.com>
Date:   Tue, 24 May 2022 09:59:30 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Song Liu <song@...nel.org>, Donald Buczek <buczek@...gen.mpg.de>
Cc:     open list <linux-kernel@...r.kernel.org>,
        linux-raid <linux-raid@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        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 v1 12/15] md/raid5-cache: Add RCU protection to conf->log
 accesses



On 2022-05-23 00:47, Song Liu wrote:
>> A problem here is that `struct r5l_log` of `conf->log` is private to raid5-cache.c and gcc below version 10 (wrongly) regards the `typeof(*p) *local` declaration of __rcu_access_pointer as a dereference:
>>
>>    CC      drivers/md/raid5.o
>>
>> In file included from ./include/linux/rculist.h:11:0,
>>
>>                   from ./include/linux/dcache.h:8,
>>
>>                   from ./include/linux/fs.h:8,
>>
>>                   from ./include/linux/highmem.h:5,
>>
>>                   from ./include/linux/bvec.h:10,
>>
>>                   from ./include/linux/blk_types.h:10,
>>
>>                   from ./include/linux/blkdev.h:9,
>>
>>                   from drivers/md/raid5.c:38:
>>
>> drivers/md/raid5-log.h: In function ‘log_stripe’:
>>
>> ./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)
>>
>>                                 ^~~~~~~~~~~~~~~~~~~~
>>
>> drivers/md/raid5-log.h:61:6: note: in expansion of macro ‘rcu_access_pointer’
>>
>>    if (rcu_access_pointer(conf->log)) {
>>
>>        ^~~~~~~~~~~~~~~~~~
>>
>> make[2]: *** [scripts/Makefile.build:288: drivers/md/raid5.o] Error 1
>>
>> make[1]: *** [scripts/Makefile.build:550: drivers/md] Error 2
>>
>> make: *** [Makefile:1834: drivers] Error 2
> 
> This is annoying.. And there are a few other cases in raid5-log.h and
> raid5.c.
> 
> Maybe we should move the definition of r5l_log to raid5-log.h?

That's the only solution I can think of, and what I'll likely do for v2.
If anyone has a better solution I'm open to it.

Logan