RE: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from atomic_t to refcount_t
From: Reshetova, Elena
Date: Tue Feb 21 2017 - 11:06:43 EST
> 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.
> > @@ -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_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.