On 07/17/2013 05:12 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()) :)
Good point Andrew, it'd be better to "document" the restriction (since
the use is non-sensical). It's actually not CFQ code that does this,
it's the io context management.
Excuse the crappy mailer, but something ala:
diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index 9c4bb82..bcb9b17 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -366,7 +366,7 @@ struct io_cq *ioc_create_icq(struct io_context *ioc,
struct
if (!icq)
return NULL;
- if (radix_tree_preload(gfp_mask) < 0) {
+ if ((gfp_mask & __GFP_WAIT) && radix_tree_preload(gfp_mask) < 0) {
kmem_cache_free(et->icq_cache, icq);
return NULL;
}
@@ -394,7 +394,10 @@ struct io_cq *ioc_create_icq(struct io_context
*ioc, struct
spin_unlock(&ioc->lock);
spin_unlock_irq(q->queue_lock);
- radix_tree_preload_end();
+
+ if (gfp_mask & __GFP_WAIT)
+ radix_tree_preload_end();
+
return icq;
}
--
Jens Axboe
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