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Re: [PATCH v2 2.6.38-rc8-tip 7/20] 7: uprobes: store/restore original instruction.
- From: Balbir Singh <balbir at linux dot vnet dot ibm dot com>
- To: Peter Zijlstra <peterz at infradead dot org>
- Cc: Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, Stephen Wilson <wilsons at start dot ca>, Ingo Molnar <mingo at elte dot hu>, Steven Rostedt <rostedt at goodmis dot org>, Linux-mm <linux-mm at kvack dot org>, Arnaldo Carvalho de Melo <acme at infradead dot org>, Linus Torvalds <torvalds at linux-foundation dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Christoph Hellwig <hch at infradead dot org>, Andi Kleen <andi at firstfloor dot org>, Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>, Oleg Nesterov <oleg at redhat dot com>, LKML <linux-kernel at vger dot kernel dot org>, Jim Keniston <jkenisto at linux dot vnet dot ibm dot com>, Roland McGrath <roland at hack dot frob dot com>, SystemTap <systemtap at sources dot redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>
- Date: Wed, 16 Mar 2011 00:28:41 +0530
- Subject: Re: [PATCH v2 2.6.38-rc8-tip 7/20] 7: uprobes: store/restore original instruction.
- References: <20110314133403.27435.7901.sendpatchset@localhost6.localdomain6> <20110314133522.27435.45121.sendpatchset@localhost6.localdomain6> <20110314180914.GA18855@fibrous.localdomain> <20110315092247.GW24254@linux.vnet.ibm.com> <1300211862.2203.302.camel@twins>
- Reply-to: balbir at linux dot vnet dot ibm dot com
* Peter Zijlstra <peterz@infradead.org> [2011-03-15 18:57:42]:
> On Tue, 2011-03-15 at 14:52 +0530, Srikar Dronamraju wrote:
> > * Stephen Wilson <wilsons@start.ca> [2011-03-14 14:09:14]:
> >
> > > On Mon, Mar 14, 2011 at 07:05:22PM +0530, Srikar Dronamraju wrote:
> > > > static int install_uprobe(struct mm_struct *mm, struct uprobe *uprobe)
> > > > {
> > > > - int ret = 0;
> > > > + struct task_struct *tsk;
> > > > + int ret = -EINVAL;
> > > >
> > > > - /*TODO: install breakpoint */
> > > > - if (!ret)
> > > > + get_task_struct(mm->owner);
> > > > + tsk = mm->owner;
> > > > + if (!tsk)
> > > > + return ret;
> > >
> > > I think you need to check that tsk != NULL before calling
> > > get_task_struct()...
> > >
> >
> > Guess checking for tsk != NULL would only help if and only if we are doing
> > within rcu. i.e we have to change to something like this
> >
> > rcu_read_lock()
> > if (mm->owner) {
> > get_task_struct(mm->owner)
> > tsk = mm->owner;
> > }
> > rcu_read_unlock()
> > if (!tsk)
> > return ret;
>
> so the whole mm->owner semantics seem vague, memcontrol.c doesn't seem
> consistent in itself, one site uses rcu_dereference() the other site
> doesn't.
>
mm->owner should be under rcu_read_lock, unless the task is exiting
and mm_count is 1. mm->owner is updated under task_lock().
> Also, the assignments in kernel/fork.c and kernel/exit.c don't use
> rcu_assign_pointer() and therefore lack the needed write barrier.
>
Those are paths when the only context using the mm->owner is single
> Git blames Balbir for this.
I accept the blame and am willing to fix anything incorrect found in
the code.
--
Three Cheers,
Balbir