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Date: Wed, 17 Jul 2013 16:16:02 -0700
From: David Daney <ddaney.cavm@gmail.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Jan Kara <jack@suse.cz>, LKML <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org, Jens Axboe <jaxboe@fusionio.com>
Subject: Re: [PATCH RFC] lib: Make radix_tree_node_alloc() irq safe
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On 07/17/2013 04:12 PM, Andrew Morton wrote:
> On Tue, 16 Jul 2013 19:06:30 +0200 Jan Kara <jack@suse.cz> 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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