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

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Message-ID: <20150826115225.GF13340@dhcppc13.redhat.com>
Date:	Wed, 26 Aug 2015 17:22:25 +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 '->'

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
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