Re: [PATCH RFC] lib: Make radix_tree_node_alloc() irq safe

[David Daney]

On 07/17/2013 04:12 PM, Andrew Morton wrote:
> On Tue, 16 Jul 2013 19:06:30 +0200 Jan Kara <jack@xxxxxxx> wrote:
>
> > With users of radix_tree_preload() run from interrupt (CFQ is one such
> > possible user), the following race can happen:
> >
> > radix_tree_preload()
> > ...
> > radix_tree_insert()
> >    radix_tree_node_alloc()
> >      if (rtp->nr) {
> >        ret = rtp->nodes[rtp->nr - 1];
> > <interrupt>
> > ...
> > radix_tree_preload()
> > ...
> > radix_tree_insert()
> >    radix_tree_node_alloc()
> >      if (rtp->nr) {
> >        ret = rtp->nodes[rtp->nr - 1];
> >
> > And we give out one radix tree node twice. That clearly results in radix
> > tree corruption with different results (usually OOPS) depending on which
> > two users of radix tree race.
> >
> > Fix the problem by disabling interrupts when working with rtp variable.
> > In-interrupt user can still deplete our preloaded nodes but at least we
> > won't corrupt radix trees.
> >
> > ...
> >
> >    There are some questions regarding this patch:
> > Do we really want to allow in-interrupt users of radix_tree_preload()?  CFQ
> > could certainly do this in older kernels but that particular call site where I
> > saw the bug hit isn't there anymore so I'm not sure this can really happen with
> > recent kernels.
>
> Well, it was never anticipated that interrupt-time code would run
> radix_tree_preload().  The whole point in the preloading was to be able
> to perform GFP_KERNEL allocations before entering the spinlocked region
> which needs to allocate memory.
>
> Doing all that from within an interrupt is daft, because the interrupt code
> can't use GFP_KERNEL anyway.
>
> > Also it is actually harmful to do preloading if you are in interrupt context
> > anyway. The disadvantage of disallowing radix_tree_preload() in interrupt is
> > that we would need to tweak radix_tree_node_alloc() to somehow recognize
> > whether the caller wants it to use preloaded nodes or not and that callers
> > would have to get it right (although maybe some magic in radix_tree_preload()
> > could handle that).
> >
> > Opinions?
>
> BUG_ON(in_interrupt()) :)

Is is really that severe?  How about...

WARN_ON() instead?

David Daney


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