linux-kernel - Re: [GIT PULL] two more s390 bug fixes for 4.17

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Message-ID: <20180531161400.GB10203@infradead.org>
Date:   Thu, 31 May 2018 09:14:00 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [GIT PULL] two more s390 bug fixes for 4.17

On Wed, May 30, 2018 at 11:11:40AM +0200, Martin Schwidefsky wrote:
> On Wed, 30 May 2018 00:41:30 -0700
> Christoph Hellwig <hch@...radead.org> wrote:
> 
> > > -	req->completion_data = cqr;
> > > +	*((struct dasd_ccw_req **) blk_mq_rq_to_pdu(req)) = cqr;
> > > +  
> > 
> > Please don't play such tricks.  In general your driver structure
> > should have struct request embedded.  If for some reason
> > struct dasd_ccw_req has a different life time please create a new
> > structure instead of these hacks.
> 
> Why do you consider this to be a 'trick'? The blk_mq_rq_to_pdu is
> meant to be used to access a block of data that is is associated
> with a request, no? With the change we store a single value, the
> pointer to a struct dasd_ccw_req. The struct request comes first,
> later do_dasd_request creates the struct dasd_ccw_req with the
> CCW chain for the request. And for the blk timeout handler we need
> a way to find the dasd_ccw_req again.

We'll we try really hard to have a structure that we can use
container_of on.  At least a minimal container with just the pointer
for a quick fix, but in general it seems like you should be able
to allocate the whole dasd_ccw_req with the request and just initialize
it later.