linux-kernel - Re: [PATCH 1/2] net: sunrpc: fix tracepoint Warning: unknown op '->'

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Message-ID: <20150826121138.GG13340@dhcppc13.redhat.com>
Date:	Wed, 26 Aug 2015 17:41:38 +0530
From:	Pratyush Anand <panand@...hat.com>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	bfields@...ldses.org, rostedt@...dmis.org,
	linux-nfs@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	"J. Bruce Fields" <bfields@...hat.com>,
	Jeff Layton <jlayton@...marydata.com>,
	open list <linux-kernel@...r.kernel.org>,
	Trond Myklebust <trond.myklebust@...marydata.com>
Subject: Re: [PATCH 1/2] net: sunrpc: fix tracepoint Warning: unknown op '->'

On 26/08/2015:08:01:21 AM, Jeff Layton wrote:
> On Wed, 26 Aug 2015 17:22:25 +0530
> Pratyush Anand <panand@...hat.com> wrote:
> 
> > Hi Jeff,
> > 
> > Thanks for your review comments.
> > 
> > On 26/08/2015:06:31:03 AM, Jeff Layton wrote:
> > > On Tue, 25 Aug 2015 11:34:19 +0530
> > > Pratyush Anand <panand@...hat.com> wrote:
> > > >  	TP_fast_assign(
> > > >  		__entry->xprt = xprt;
> > > > +		__entry->addr =
> > > > +			xprt ? (struct sockaddr *)&xprt->xpt_remote : NULL;
> > > 
> > > I don't get it. It's not safe to save a pointer to xprt and dereference
> > > that in a tracepoint (and I understand why that is), but it is safe to
> > > save a pointer to a structure embedded inside of xprt? Shouldn't you be
> > > saving a copy of the entire sockaddr struct instead?
> > 
> > As far as "saving of a pointer to a structure embedded inside of xprt" is
> > concerned, I do not see any issue.
> > 
> > Its your take that what you want to print as your tracepoint print output. I
> > might be missing something..  However, I do not see any value addition in
> > printing address of a structure(located in ring buffer) where xprt->xpt_remote
> > has been copied.
> > 
> > > 
> > > Ahh, ok -- I think I see the confusion. %pIScp does not print the
> > > address of the sockaddr, but instead dereferences the pointer and
> > > prints it as a formatted address string. See pointer() in
> > > lib/vsprintf.c. You do want to save off a copy of the structure instead.
> > 
> > In my opinion, saving of structure would only be necessary if you want to access
> > element of the structure xprt->xpt_remote.
> > 
> > ~Pratyush
> 
> That's exactly what %pI does. Look at the pointer() function.

My bad..am a slow learner :(
Will send V2. Thanks for explaining.

~Pratyush
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