Re: [PATCH 3/7] fs, xfs: convert xfs_buf_log_item.bli_refcount from atomic_t to refcount_t
From: Peter Zijlstra
Date: Tue Feb 21 2017 - 11:28:29 EST
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.
For FS changes, I would recommend at the very least running xfstests on
an instance (which unlike the name suggests is good for most block
filesystems Linux has).
> > > @@ -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.
No, your conversion should not generate spurious WARN()s.
And also no, atomic_dec() must not hit 0. If you've been _that_ careful
with your reference counting and you absolutely _know_ this is the very
last one, write something like:
Please, stop sending out conversions that haven't been tested. And take
the time to actually look at your own patches. If I can spot fail just
looking through them, so can you (or any of the other many people in
your SoB chain).
Take a little more time and a little extra care.