linux-kernel - Re: [RFC PATCH v3 4/6] uretprobes: return probe entry, prepare uretprobe

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Date:	Mon, 4 Mar 2013 15:14:19 +0100
From:	Anton Arapov <anton@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Stone <jistone@...hat.com>,
	Frank Eigler <fche@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [RFC PATCH v3 4/6] uretprobes: return probe entry, prepare
 uretprobe

On Thu, Feb 28, 2013 at 09:10:24PM +0100, Oleg Nesterov wrote:
> On 02/28, Anton Arapov wrote:
> >
> > +static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
> > +{
> > +	struct return_uprobe_i *ri;
> > +	struct uprobe_task *utask;
> > +	struct xol_area *area;
> > +	unsigned long rp_trampoline_vaddr = 0;
> > +	uprobe_opcode_t insn = UPROBE_SWBP_INSN;
> > +
> > +	area = get_xol_area();
> > +	if (area)
> > +		rp_trampoline_vaddr = area->rp_trampoline_vaddr;
> > +	if (!rp_trampoline_vaddr) {
> > +		rp_trampoline_vaddr = xol_get_insn_slot(&insn);
> > +		if (!rp_trampoline_vaddr)
> > +			return;
> > +	}
> > +	area->rp_trampoline_vaddr = rp_trampoline_vaddr;
> 
> This is called under down_read(), so 2 threads can race with each other
> and use the different rp_trampoline_vaddr's if ->rp_trampoline_vaddr was
> NULL.
> And again, I think ->rp_trampoline_vaddr is simply unneeded, see my
> reply to 3/6.
  Yes, 'fixed' this, I will send quirky v4 in a few, to maintain your attention. :)

> 
> >  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> >  {
> > +	int rc = 0;
> >  	struct uprobe_consumer *uc;
> >  	int remove = UPROBE_HANDLER_REMOVE;
> >  
> >  	down_read(&uprobe->register_rwsem);
> >  	for (uc = uprobe->consumers; uc; uc = uc->next) {
> > -		int rc = uc->handler(uc, regs);
> > +		if (uc->handler)
> > +			rc = uc->handler(uc, regs);
> > +
> > +		if (uc->rp_handler)
> > +			prepare_uretprobe(uprobe, regs); /* put bp at return */
> 
> Hmm. I didn't read this series yet. But at first glance I am not
> sure prepare_uretprobe() should be called every time we see
> ->rp_handler != NULL, there could be multiple consumers...

  I can not come up with a better idea yet, that would narrow down the
number of prepare_uretprobe() calls. 

Anton.
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