Re: [PATCH 3/8] mm, hugetlb: fix race in region tracking

[Davidlohr Bueso]

On Mon, 2014-01-27 at 16:02 -0500, Naoya Horiguchi wrote:
> On Sun, Jan 26, 2014 at 07:52:21PM -0800, Davidlohr Bueso wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > 
> > There is a race condition if we map a same file on different processes.
> > Region tracking is protected by mmap_sem and hugetlb_instantiation_mutex.
> > When we do mmap, we don't grab a hugetlb_instantiation_mutex, but only the,
> > mmap_sem (exclusively). This doesn't prevent other tasks from modifying the
> > region structure, so it can be modified by two processes concurrently.
> > 
> > To solve this, introduce a spinlock to resv_map and make region manipulation
> > function grab it before they do actual work.
> > 
> > Acked-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > [Updated changelog]
> > Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> > ---
> ...
> > @@ -203,15 +200,23 @@ static long region_chg(struct resv_map *resv, long f, long t)
> >  	 * Subtle, allocate a new region at the position but make it zero
> >  	 * size such that we can guarantee to record the reservation. */
> >  	if (&rg->link == head || t < rg->from) {
> > -		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> > -		if (!nrg)
> > -			return -ENOMEM;
> > +		if (!nrg) {
> > +			spin_unlock(&resv->lock);
> 
> I think that doing kmalloc() inside the lock is simpler.
> Why do you unlock and retry here?

This is a spinlock, no can do -- we've previously debated this and since
the critical region is quite small, a non blocking lock is better suited
here. We do the retry so we don't race once the new region is allocated
after the lock is dropped.

Thanks,
Davidlohr

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