linux-kernel - Re: [PATCH 4/5] fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to refcount

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Date:   Tue, 7 Mar 2017 12:30:15 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "david@...morbit.com" <david@...morbit.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 4/5] fs, xfs: convert xfs_cui_log_item.cui_refcount from
 atomic_t to refcount_t

On Tue, Mar 07, 2017 at 05:33:14PM +0000, Reshetova, Elena wrote:
>  > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> > > Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
> > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > Signed-off-by: David Windsor <dwindsor@...il.com>
> > > ---
> > >  fs/xfs/xfs_refcount_item.c | 4 ++--
> > >  fs/xfs/xfs_refcount_item.h | 4 +++-
> > >  2 files changed, 5 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_refcount_item.c b/fs/xfs/xfs_refcount_item.c
> > > index 6e4c744..61bc570 100644
> > > --- a/fs/xfs/xfs_refcount_item.c
> > > +++ b/fs/xfs/xfs_refcount_item.c
> > > @@ -205,7 +205,7 @@ xfs_cui_init(
> > >  	cuip->cui_format.cui_nextents = nextents;
> > >  	cuip->cui_format.cui_id = (uintptr_t)(void *)cuip;
> > >  	atomic_set(&cuip->cui_next_extent, 0);
> > > -	atomic_set(&cuip->cui_refcount, 2);
> > > +	refcount_set(&cuip->cui_refcount, 2);
> > 
> > I'm assuming the refcount design is ok with the log item refcounts
> > starting at 2 and marching down to zero?  The code seems to be
> > fine with it; I just want to make sure everyone's ok with supporting
> > this use case.
> 
> Yes, as soon as it doesn't go beyond 0 or starting to increment again after reaching zero. 

Ok, good.

> > 
> > >  	return cuip;
> > >  }
> > > @@ -221,7 +221,7 @@ void
> > >  xfs_cui_release(
> > >  	struct xfs_cui_log_item	*cuip)
> > >  {
> > > -	if (atomic_dec_and_test(&cuip->cui_refcount)) {
> > > +	if (refcount_dec_and_test(&cuip->cui_refcount)) {
> > 
> > I suppose it's useful to have refcount_dec_and_test complain loudly if
> > we ever have a dangling pointer... Friday I was chasing a possible
> > use-after-free of the EFI items in generic/388.
> 
> Currently it complains with WARN if it finds itself decrementing from zero. 
> There is patch in RFC stage also that would make it configurable: complain with WARN or complain/abort with BUG. 
> 
> > 
> > >  		xfs_trans_ail_remove(&cuip->cui_item,
> > SHUTDOWN_LOG_IO_ERROR);
> > >  		xfs_cui_item_free(cuip);
> > >  	}
> > > diff --git a/fs/xfs/xfs_refcount_item.h b/fs/xfs/xfs_refcount_item.h
> > > index 5b74ddd..7f23ff8 100644
> > > --- a/fs/xfs/xfs_refcount_item.h
> > > +++ b/fs/xfs/xfs_refcount_item.h
> > > @@ -20,6 +20,8 @@
> > >  #ifndef	__XFS_REFCOUNT_ITEM_H__
> > >  #define	__XFS_REFCOUNT_ITEM_H__
> > >
> > > +#include <linux/refcount.h>
> > 
> > I still think this include should go in xfs_linux.h, the same as most of
> > the other linux/*.h includes in XFS.
> 
> Oh, very sorry, I forgot this one. Got too much focused on testing :(
> Do you want me to resend with this change or can you take it in and do this small fix yourself? 

Since the comment applies to all the patches, please resend the series
with all the patches fixed up.

--D

> 
> Best Regards,
> Elena.
> 
> 
> > 
> > --D
> > 
> > > +
> > >  /*
> > >   * There are (currently) two pairs of refcount btree redo item types:
> > >   * increase and decrease.  The log items for these are CUI (refcount
> > > @@ -63,7 +65,7 @@ struct kmem_zone;
> > >   */
> > >  struct xfs_cui_log_item {
> > >  	struct xfs_log_item		cui_item;
> > > -	atomic_t			cui_refcount;
> > > +	refcount_t			cui_refcount;
> > >  	atomic_t			cui_next_extent;
> > >  	unsigned long			cui_flags;	/* misc
> > flags */
> > >  	struct xfs_cui_log_format	cui_format;
> > > --
> > > 2.7.4
> > >
> > > --
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