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

[Jens Axboe]

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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