linux-kernel - Re: [PATCH v2] block: fix mis-synchronisation in blkdev_issue

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Date:	Fri, 11 Mar 2011 15:31:15 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Jens Axboe <axboe@...nel.dk>,
	Dmitry Monakhov <dmonakhov@...nvz.org>
Subject: Re: [PATCH v2] block: fix mis-synchronisation in
 blkdev_issue_zeroout()

On Fri, 11 Mar 2011, Jeff Moyer wrote:

> Lukas Czerner <lczerner@...hat.com> writes:
> 
> > BZ29402
> > https://bugzilla.kernel.org/show_bug.cgi?id=29402
> >
> > We can hit serious mis-synchronization in bio completion path of
> > blkdev_issue_zeroout() leading to a panic.
> >
> > The problem is that when we are going to wait_for_completion() in
> > blkdev_issue_zeroout() we check if the bb.done equals issued (number of
> > submitted bios). If it does, we can skip the wait_for_completition()
> > and just out of the function since there is nothing to wait for.
> > However, there is a ordering problem because bio_batch_end_io() is
> > calling atomic_inc(&bb->done) before complete(), hence it might seem to
> > blkdev_issue_zeroout() that all bios has been completed and exit. At
> > this point when bio_batch_end_io() is going to call complete(bb->wait),
> > bb and wait does not longer exist since it was allocated on stack in
> > blkdev_issue_zeroout() ==> panic!
> >
> > (thread 1)                      (thread 2)
> > bio_batch_end_io()              blkdev_issue_zeroout()
> >   if(bb) {                      ...
> >     if (bb->end_io)             ...
> >       bb->end_io(bio, err);     ...
> >     atomic_inc(&bb->done);      ...
> >     ...                         while (issued != atomic_read(&bb.done))
> >     ...                         (let issued == bb.done)
> >     ...                         (do the rest of the function)
> >     ...                         return ret;
> >     complete(bb->wait);
> >     ^^^^^^^^
> >     panic
> >
> > We can fix this easily by simplifying bio_batch and completion counting.
> >
> > Also remove bio_end_io_t *end_io since it is not used.
> >
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > Reported-by: Eric Whitney <eric.whitney@...com>
> > Tested-by: Eric Whitney <eric.whitney@...com>
> > CC: Jens Axboe <axboe@...nel.dk>
> > CC: Dmitry Monakhov <dmonakhov@...nvz.org>
> > CC: Jeff Moyer <jmoyer@...hat.com>
> > ---
> >  block/blk-lib.c |   19 +++++++------------
> >  1 files changed, 7 insertions(+), 12 deletions(-)
> >
> > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > index eec78be..bd3e8df 100644
> > --- a/block/blk-lib.c
> > +++ b/block/blk-lib.c
> > @@ -109,7 +109,6 @@ struct bio_batch
> >  	atomic_t 		done;
> >  	unsigned long 		flags;
> >  	struct completion 	*wait;
> > -	bio_end_io_t		*end_io;
> >  };
> >  
> >  static void bio_batch_end_io(struct bio *bio, int err)
> > @@ -122,12 +121,9 @@ static void bio_batch_end_io(struct bio *bio, int err)
> >  		else
> >  			clear_bit(BIO_UPTODATE, &bb->flags);
> >  	}
> > -	if (bb) {
> > -		if (bb->end_io)
> > -			bb->end_io(bio, err);
> > -		atomic_inc(&bb->done);
> > -		complete(bb->wait);
> > -	}
> > +	if (bb)
> > +		if (atomic_dec_and_test(&bb->done))
> > +			complete(bb->wait);
> 
> I think bb will always be set here, no real need to check.
> 
> Anyway, I though I already added my:
> 
> Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
> 
> to this.  No?
> 
> Cheers,
> Jeff

Yes, you did and I forgot to add it into the patch. Sorry about that.

Thanks!
-Lukas
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