linux-kernel - Re: [PATCHv11 2.6.36-rc2-tip 3/15] 3: uprobes: Slot allocation for Execution out of line(XOL)

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Date:	Fri, 03 Sep 2010 19:41:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Randy Dunlap <rdunlap@...otime.net>,
	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>,
	Oleg Nesterov <oleg@...hat.com>,
	Mark Wielaard <mjw@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naren A Devaiah <naren.devaiah@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCHv11 2.6.36-rc2-tip 3/15]  3: uprobes: Slot allocation
 for Execution out of line(XOL)

On Fri, 2010-09-03 at 22:56 +0530, Srikar Dronamraju wrote:
> 
> 
> static unsigned long xol_get_insn_slot(struct user_bkpt *user_bkpt,
>                                 struct uprobes_xol_area *xol_area)
> {
>         unsigned long flags, xol_vaddr = 0;
>         int len;
> 
>         if (unlikely(!xol_area))
>                 return 0;
> 
>         smp_rmb();
>         if (user_bkpt->xol_vaddr)
>                 return user_bkpt->xol_vaddr;
> 
>         spin_lock_irqsave(&xol_area->lock, flags);
>         xol_vaddr = xol_take_insn_slot(xol_area);
>         spin_unlock_irqrestore(&xol_area->lock, flags);
> 
>         /*
>          * Initialize the slot if user_bkpt->vaddr points to valid
>          * instruction slot.
>          */
>         if (!xol_vaddr)
>                 return 0;
> 
>         len = access_process_vm(current, xol_vaddr, user_bkpt->insn,
>                                         UPROBES_XOL_SLOT_BYTES, 1);
>         if (unlikely(len < UPROBES_XOL_SLOT_BYTES))
>                 printk(KERN_ERR "Failed to copy instruction at %#lx "
>                                 "len = %d\n", user_bkpt->vaddr, len);
> 
>         /*
>          * Update user_bkpt->xol_vaddr after giving a chance for the slot to
>          * be initialized.
>          */
>         smp_mb();
>         user_bkpt->xol_vaddr = xol_vaddr;
>         return user_bkpt->xol_vaddr;
> } 

Racy like you won't believe..

Suppose multiple threads hitting the trap at the same time, every thread
will end up failing the check and allocating a new slot for it, at the
end the slowest thread will end up setting the value.


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