linux-kernel - Re: [PATCH v2 2.6.38-rc8-tip 5/20] 5: Uprobes: register/unregister probes.

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Date:	Tue, 15 Mar 2011 23:34:23 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 2.6.38-rc8-tip 5/20] 5: Uprobes: register/unregister
 probes.

* Peter Zijlstra <peterz@...radead.org> [2011-03-15 18:50:11]:

> On Tue, 2011-03-15 at 13:47 -0400, Steven Rostedt wrote:
> > On Tue, 2011-03-15 at 22:45 +0530, Srikar Dronamraju wrote:
> > > > > +   }
> > > > > +   list_for_each_entry_safe(mm, tmpmm, &tmp_list, uprobes_list) {
> > > > > +           down_read(&mm->mmap_sem);
> > > > > +           if (!install_uprobe(mm, uprobe))
> > > > > +                   ret = 0;
> > > > 
> > > > Installing it once is success ?
> > > 
> > > This is a little tricky. My intention was to return success even if one
> > > install is successful. If we return error, then the caller can go
> > > ahead and free the consumer. Since we return success if there are
> > > currently no processes that have mapped this inode, I was tempted to
> > > return success on atleast one successful install.
> > 
> > What about an all or nothing approach. If one fails, remove all that
> > were installed, and give up.
> 
> That sounds like a much saner semantic and is what we generally do in
> the kernel.

One of the install_uprobe could be failing because the process was
almost exiting, something like there was no mm->owner. Also lets
assume that the first few install_uprobes go thro and the last
install_uprobe fails. There could be breakpoint hits corresponding to
the already installed uprobes that get displayed. i.e all
breakpoint hits from the first install_uprobe to the time we detect a
failed a install_uprobe and revert all inserted breakpoints will be
shown as being captured.

Also register_uprobe will only install probes for processes that are
actively and have mapped the inode.  However install_uprobe called from
uprobe_mmap which also registers the probe can fail. Now should we take
the same yardstick and remove all the probes corresponding to the
successful register_uprobe?
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