Thank you for pointing out the problem in the patch. I cannot find an unique refcount API work like atomic_inc_return, thus I chose two APIs to play a similar role and forgot the potential racy case. So are you have a better choice to help this refcount type convertation?
> -----Original Messages-----
> From: "Jeff Layton" <jlayton@xxxxxxxxxx>
> Sent Time: 2021-07-17 19:21:40 (Saturday)
> To: "Xiyu Yang" <xiyuyang19@xxxxxxxxxxxx>, "Ilya Dryomov" <idryomov@xxxxxxxxx>, ceph-devel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Cc: yuanxzhang@xxxxxxxxxxxx, "Xin Tan" <tanxin.ctf@xxxxxxxxx>, "Yejune Deng" <yejune.deng@xxxxxxxxx>
> Subject: Re: [PATCH] ceph: Convert from atomic_t to refcount_t on ceph_snap_realm->nref
>
> On Sat, 2021-07-17 at 18:06 +0800, Xiyu Yang wrote:
> > refcount_t type and corresponding API can protect refcounters from
> > accidental underflow and overflow and further use-after-free situations.
> >
> > Signed-off-by: Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>
> > Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>
> > ---
> > fs/ceph/snap.c | 15 ++++++++-------
> > fs/ceph/super.h | 3 ++-
> > 2 files changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> > index 4ac0606dcbd4..d4ec9c5118bd 100644
> > --- a/fs/ceph/snap.c
> > +++ b/fs/ceph/snap.c
> > @@ -68,14 +68,15 @@ void ceph_get_snap_realm(struct ceph_mds_client *mdsc,
> > lockdep_assert_held(&mdsc->snap_rwsem);
> >
> > dout("get_realm %p %d -> %d\n", realm,
> > - atomic_read(&realm->nref), atomic_read(&realm->nref)+1);
> > + refcount_read(&realm->nref), refcount_read(&realm->nref)+1);
> > /*
> > * since we _only_ increment realm refs or empty the empty
> > * list with snap_rwsem held, adjusting the empty list here is
> > * safe. we do need to protect against concurrent empty list
> > * additions, however.
> > */
> > - if (atomic_inc_return(&realm->nref) == 1) {
> > + refcount_inc(&realm->nref);
> > + if (refcount_read(&realm->nref) == 1) {
>
> The above is potentially racy as you've turned a single atomic operation
> into two. Another task could come in and increment or decrement
> realm->nref just after your recount_inc but before the refcount_read,
> and then the read would show the wrong result.
>
> FWIW, Yejune Deng (cc'ed) proposed a very similar patch a few months ago
> that caused this regression:
>
> https://tracker.ceph.com/issues/50281
>
> > spin_lock(&mdsc->snap_empty_lock);
> > list_del_init(&realm->empty_item);
> > spin_unlock(&mdsc->snap_empty_lock);
> > @@ -121,7 +122,7 @@ static struct ceph_snap_realm *ceph_create_snap_realm(
> > if (!realm)
> > return ERR_PTR(-ENOMEM);
> >
> > - atomic_set(&realm->nref, 1); /* for caller */
> > + refcount_set(&realm->nref, 1); /* for caller */
> > realm->ino = ino;
> > INIT_LIST_HEAD(&realm->children);
> > INIT_LIST_HEAD(&realm->child_item);
> > @@ -209,8 +210,8 @@ static void __put_snap_realm(struct ceph_mds_client *mdsc,
> > lockdep_assert_held_write(&mdsc->snap_rwsem);
> >
> > dout("__put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
> > - atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
> > - if (atomic_dec_and_test(&realm->nref))
> > + refcount_read(&realm->nref), refcount_read(&realm->nref)-1);
> > + if (refcount_dec_and_test(&realm->nref))
> > __destroy_snap_realm(mdsc, realm);
> > }
> >
> > @@ -221,8 +222,8 @@ void ceph_put_snap_realm(struct ceph_mds_client *mdsc,
> > struct ceph_snap_realm *realm)
> > {
> > dout("put_snap_realm %llx %p %d -> %d\n", realm->ino, realm,
> > - atomic_read(&realm->nref), atomic_read(&realm->nref)-1);
> > - if (!atomic_dec_and_test(&realm->nref))
> > + refcount_read(&realm->nref), refcount_read(&realm->nref)-1);
> > + if (!refcount_dec_and_test(&realm->nref))
> > return;
> >
> > if (down_write_trylock(&mdsc->snap_rwsem)) {
> > diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> > index 6b6332a5c113..3abb00d7a0eb 100644
> > --- a/fs/ceph/super.h
> > +++ b/fs/ceph/super.h
> > @@ -2,6 +2,7 @@
> > #ifndef _FS_CEPH_SUPER_H
> > #define _FS_CEPH_SUPER_H
> >
> > +#include <linux/refcount.h>
> > #include <linux/ceph/ceph_debug.h>
> >
> > #include <asm/unaligned.h>
> > @@ -859,7 +860,7 @@ struct ceph_readdir_cache_control {
> > struct ceph_snap_realm {
> > u64 ino;
> > struct inode *inode;
> > - atomic_t nref;
> > + refcount_t nref;
> > struct rb_node node;
> >
> > u64 created, seq;
>
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
