Re: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from atomic_t to refcount_t

From: Brian Foster
Date: Tue Feb 21 2017 - 14:27:27 EST


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.

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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