Re: [PATCH] dio: remove bogus refcounting BUG_ON

[Suparna Bhattacharya]

On Wed, Jul 04, 2007 at 07:25:10PM -0700, Badari Pulavarty wrote:
> On Tue, 2007-07-03 at 15:28 -0700, Zach Brown wrote:
> > Linus, Andrew, please apply the bug fix patch at the end of this reply
> > for .22.
> > 
> > > >>One of our perf. team ran into this while doing some runs.
> > > >>I didn't see anything obvious - it looks like we converted
> > > >>async IO to synchronous one. I didn't spend much time digging
> > > >>around.
> > 
> > OK, I think this BUG_ON() is just broken.  I wasn't able to find any
> > obvious bugs from reading the code which would cause the BUG_ON() to
> > fire.  If it's reproducible I'd love to hear what the recipe is.
> > 
> > I did notice that this BUG_ON() is evaluating dio after having dropped
> > it's ref :/.  So it's not completely absurd to fear that it's a race
> > with the dio's memory being reused, but that'd be a pretty tight race.
> > 
> > Let's remove this stupid BUG_ON and see if that test box still has
> > trouble.  It might just hit the valid BUG_ON a few lines down, but this
> > unsafe BUG_ON needs to go.
> 
> I went through the code multiple times, I can't find how we can trigger
> the BUG_ON(). But unfortunately, our perf. team is able reproduce the
> problem. Debug indicated that, the ret2 == 1 :(
> 
> Not sure how that can happen. Ideas ?

Does it trigger even if you avoid referencing dio in the BUG_ON(), i.e.
with something like ...


--- direct-io.c	2007-07-02 01:24:24.000000000 +0530
+++ direct-io-debug.c	2007-07-05 09:18:56.000000000 +0530
@@ -1104,9 +1104,10 @@ direct_io_worker(int rw, struct kiocb *i
 	 * decide to wake the submission path atomically.
 	 */
 	spin_lock_irqsave(&dio->bio_lock, flags);
+	is_async = dio->is_async;
 	ret2 = --dio->refcount;
 	spin_unlock_irqrestore(&dio->bio_lock, flags);
-	BUG_ON(!dio->is_async && ret2 != 0);
+	BUG_ON(!is_async && ret2 != 0);
 	if (ret2 == 0) {
 		ret = dio_complete(dio, offset, ret);
 		kfree(dio);

> 
> Thanks,
> Badari
> 
> > 
> > -------
> > 
> > dio: remove bogus refcounting BUG_ON
> > 
> > Badari Pulavarty reported a case of this BUG_ON is triggering during
> > testing.  It's completely bogus and should be removed.
> > 
> > It's trying to notice if we left references to the dio hanging around in
> > the sync case.  They should have been dropped as IO completed while this
> > path was in dio_await_completion().  This condition will also be
> > checked, via some twisty logic, by the BUG_ON(ret != -EIOCBQUEUED) a few
> > lines lower.  So to start this BUG_ON() is redundant.
> > 
> > More fatally, it's dereferencing dio-> after having dropped its
> > reference.  It's only safe to dereference the dio after releasing the
> > lock if the final reference was just dropped.  Another CPU might free
> > the dio in bio completion and reuse the memory after this path drops the
> > dio lock but before the BUG_ON() is evaluated.
> > 
> > This patch passed aio+dio regression unit tests and aio-stress on ext3.
> > 
> > Signed-off-by: Zach Brown <zach.brown@xxxxxxxxxx>
> > Cc: Badari Pulavarty <pbadari@xxxxxxxxxx>
> > 
> > diff -r 509ce354ae1b fs/direct-io.c
> > --- a/fs/direct-io.c	Sun Jul 01 22:00:49 2007 +0000
> > +++ b/fs/direct-io.c	Tue Jul 03 14:56:41 2007 -0700
> > @@ -1106,7 +1106,7 @@ direct_io_worker(int rw, struct kiocb *i
> >  	spin_lock_irqsave(&dio->bio_lock, flags);
> >  	ret2 = --dio->refcount;
> >  	spin_unlock_irqrestore(&dio->bio_lock, flags);
> > -	BUG_ON(!dio->is_async && ret2 != 0);
> > +
> >  	if (ret2 == 0) {
> >  		ret = dio_complete(dio, offset, ret);
> >  		kfree(dio);
> 
> -
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-- 
Suparna Bhattacharya (suparna@xxxxxxxxxx)
Linux Technology Center
IBM Software Lab, India

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