linux-kernel - RE: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from atomic_t to refcount

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Date:   Wed, 22 Feb 2017 11:26:20 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Brian Foster <bfoster@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        "Kees Cook" <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: RE: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from
 atomic_t to refcount_t

> On Tue, Feb 21, 2017 at 09:06:20AM -0800, Darrick J. Wong wrote:
> > On Tue, Feb 21, 2017 at 04:06:30PM +0000, Reshetova, Elena wrote:
> > > > On Tue, Feb 21, 2017 at 05:49:03PM +0200, Elena Reshetova wrote:
> > > > > refcount_t type and corresponding API should be
> > > > > used instead of atomic_t when the variable is used as
> > > > > a reference counter. This allows to avoid accidental
> > > > > refcounter overflows that might lead to use-after-free
> > > > > situations.
> > > >
> > > > Changelog forgets to mention if this was runtime tested..
> > >
> > > It was boot-tested in the whole refcount_t changes pile, which is not very
> useful for fs anyway.
> > > What's why we are sending this through maintainers to get through their
> tests.
> > > I am sure that testing would be better than what we can do.
> >
> > If you're going to go around making this many changes to XFS (or any
> > other filesystem), please run the changes through xfstests first.
> > Many fs projects (not just XFS) record their test cases there.
> >
> > I think the kernel 0day build service is supposed to do that
> > automatically...
> >
> 
> Be sure to use CONFIG_XFS_DEBUG and/or CONFIG_XFS_WARN to capture
> any
> potential assert failures as well.

Thanks for pointing to this! I have been actually asking before on how to runtime test things more with all our patches before submission, but got reply around: "submit to maintainers, they know how to do it". 
I think we need to look into 0day automated testing, otherwise since we make changes to many FSes, testing this manually would be little fun...

Best Regards,
Elena
> 
> Brian
> 
> > --D
> >
> > >
> > > >
> > > >
> > > > > @@ -371,7 +371,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> > > > >  	ASSERT(bip->bli_item.li_type == XFS_LI_BUF);
> > > > >  	ASSERT(!(bip->bli_flags & XFS_BLI_STALE));
> > > > >  	ASSERT(!(bip->__bli_format.blf_flags & XFS_BLF_CANCEL));
> > > > > -	ASSERT(atomic_read(&bip->bli_refcount) > 0);
> > > > > +	ASSERT(refcount_read(&bip->bli_refcount) > 0);
> > > > >
> > > > >  	trace_xfs_trans_brelse(bip);
> > > > >
> > > > > @@ -419,7 +419,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> > > > >  	/*
> > > > >  	 * Drop our reference to the buf log item.
> > > > >  	 */
> > > > > -	atomic_dec(&bip->bli_refcount);
> > > > > +	refcount_dec(&bip->bli_refcount);
> > > > >
> > > > >  	/*
> > > > >  	 * If the buf item is not tracking data in the log, then
> > > > > @@ -432,7 +432,7 @@ xfs_trans_brelse(xfs_trans_t	*tp,
> > > > >  /***
> > > > >  		ASSERT(bp->b_pincount == 0);
> > > > >  ***/
> > > > > -		ASSERT(atomic_read(&bip->bli_refcount) == 0);
> > > > > +		ASSERT(refcount_read(&bip->bli_refcount) == 0);
> > > > >  		ASSERT(!(bip->bli_item.li_flags & XFS_LI_IN_AIL));
> > > > >  		ASSERT(!(bip->bli_flags &
> > > > XFS_BLI_INODE_ALLOC_BUF));
> > > > >  		xfs_buf_item_relse(bp);
> > > >
> > > >
> > > > This for example looks dodgy.
> > > >
> > > > That seems to suggest the atomic_dec() there can actually hit 0, which
> > > > _will_ generate a WARN.
> > >
> > > True, but in some of this cases WARN might be ok, I think? As soon as
> functionality is not changed and object is not reused (by doing refcount_inc on
> it) anywhere later on.
> > >
> > > --
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