linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

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Date:	Tue, 27 Sep 2011 14:18:52 +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>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 18/26]   uprobes: slot allocation.

On Tue, 2011-09-20 at 17:33 +0530, Srikar Dronamraju wrote:
> +static struct uprobes_xol_area *xol_alloc_area(void)
> +{
> +       struct uprobes_xol_area *area = NULL;
> +
> +       area = kzalloc(sizeof(*area), GFP_KERNEL);
> +       if (unlikely(!area))
> +               return NULL;
> +
> +       area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
> +                                                               GFP_KERNEL);
> +
> +       if (!area->bitmap)
> +               goto fail;
> +
> +       init_waitqueue_head(&area->wq);
> +       spin_lock_init(&area->slot_lock);
> +       if (!xol_add_vma(area) && !current->mm->uprobes_xol_area) {

So what happens if xol_add_vma() succeeds, but we find
->uprobes_xol_area set?

> +               task_lock(current);
> +               if (!current->mm->uprobes_xol_area) {

Having to re-test it under this lock seems to suggest it could.

> +                       current->mm->uprobes_xol_area = area;
> +                       task_unlock(current);
> +                       return area;

This function would be so much easier to read if the success case (this
here I presume) would not be nested 2 deep.

> +               }
> +               task_unlock(current);
> +       }

at which point you could end up with two extra vmas? Because there's no
freeing of the result of xol_add_vma().

> +fail:
> +       kfree(area->bitmap);
> +       kfree(area);
> +       return current->mm->uprobes_xol_area;
> +} 
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