linux-kernel - Re: [PATCH v8 3.2.0-rc5 1/9] uprobes: Install and remove breakpoints.

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Date:	Wed, 04 Jan 2012 12:12:30 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH v8 3.2.0-rc5 1/9] uprobes: Install and remove
 breakpoints.

On Wed, 2012-01-04 at 17:51 +0100, Peter Zijlstra wrote:

> > +               if (is_register)
> > +                       ret = install_breakpoint(mm, uprobe, vma, vi->vaddr);
> > +               else
> > +                       remove_breakpoint(mm, uprobe, vi->vaddr);
> > +
> > +               up_read(&mm->mmap_sem);
> > +               mmput(mm);
> > +               if (is_register) {
> > +                       if (ret && ret == -EEXIST)
> > +                               ret = 0;
> > +                       if (ret)
> > +                               break;
> > +               }
> 
> Since you init ret := 0 and remove_breakpoint doesn't change it, this
> conditional on is_register is superfluous.

True, but I would argue that this is easier to understand. That is, we
only break on a failed install_breakpoint (is_register is set). If I
looked at this code and saw:

	if (is_register)
		ret = install_breakpoint()
	else	
		remove_breakpoint()

	[...]

	if (ret && ret == -EEXIST)
		ret = 0;
	if (ret)
		break;

I would first think that there might be a bug. That is, we should have a
ret = remove_breakpoint().

Thus, I would say, either leave this as is and hope gcc is smart enough
to optimize out the if (is_register), or add the comment:

	/* ret will always be zero on remove_breakpoint */
	if (ret && ret == -EEXIST)
		ret = 0;
	if (ret)
		break;

-- Steve

> 
> > +       }
> > +       list_for_each_entry_safe(vi, tmpvi, &try_list, probe_list) {
> > +               list_del(&vi->probe_list);
> > +               kfree(vi);
> > +       }
> > +       return ret;
> > +} 


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