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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Message-ID: <20100906053847.GE14891@linux.vnet.ibm.com>
Date:	Mon, 6 Sep 2010 11:08:47 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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)

> > 
> > 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.
> 

Agree, I shall fix this up.
Since set_bit and clear_bit are atomic, I shall change the
area->lock from a spinlock to a mutex, and have the mutex released
after the slot has been updated with the "single-stepping
instruction".

--
Thanks and Regards
Srikar
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