Re: [PATCH V2] fs/ceph:fix double unlock in handle_cap_export()

[Wu Bo]

On 2020/4/30 10:50, Yan, Zheng wrote:
> On Wed, Apr 29, 2020 at 8:49 AM Wu Bo <wubo40@xxxxxxxxxx> wrote:
> > On 2020/4/28 22:48, Jeff Layton wrote:
> > > On Tue, 2020-04-28 at 21:13 +0800, Wu Bo wrote:
> > > > if the ceph_mdsc_open_export_target_session() return fails,
> > > > should add a lock to avoid twice unlocking.
> > > > Because the lock will be released at the retry or out_unlock tag.
> > >
> > > The problem looks real, but...
> > >
> > > > --
> > > > v1 -> v2:
> > > > add spin_lock(&ci->i_ceph_lock) before goto out_unlock tag.
> > > >
> > > > Signed-off-by: Wu Bo <wubo40@xxxxxxxxxx>
> > > > ---
> > > >    fs/ceph/caps.c | 27 +++++++++++++++------------
> > > >    1 file changed, 15 insertions(+), 12 deletions(-)
> > > >
> > > > diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> > > > index 185db76..414c0e2 100644
> > > > --- a/fs/ceph/caps.c
> > > > +++ b/fs/ceph/caps.c
> > > > @@ -3731,22 +3731,25 @@ static void handle_cap_export(struct inode *inode, struct ceph_mds_caps *ex,
> > > >
> > > >       /* open target session */
> > > >       tsession = ceph_mdsc_open_export_target_session(mdsc, target);
> > > > -    if (!IS_ERR(tsession)) {
> > > > -            if (mds > target) {
> > > > -                    mutex_lock(&session->s_mutex);
> > > > -                    mutex_lock_nested(&tsession->s_mutex,
> > > > -                                      SINGLE_DEPTH_NESTING);
> > > > -            } else {
> > > > -                    mutex_lock(&tsession->s_mutex);
> > > > -                    mutex_lock_nested(&session->s_mutex,
> > > > -                                      SINGLE_DEPTH_NESTING);
> > > > -            }
> > > > -            new_cap = ceph_get_cap(mdsc, NULL);
> > > > -    } else {
> > > > +    if (IS_ERR(tsession)) {
> > > >               WARN_ON(1);
> > > >               tsession = NULL;
> > > >               target = -1;
> > > > +            mutex_lock(&session->s_mutex);
> > > > +            spin_lock(&ci->i_ceph_lock);
> > > > +            goto out_unlock;
> > >
> > > Why did you make this case goto out_unlock instead of retrying as it did
> > > before?
> >
> > If the problem occurs, target = -1, and goto retry lable, you need to
> > call __get_cap_for_mds() or even call __ceph_remove_cap(), and then jump
> > to out_unlock lable. All I think is unnecessary, goto out_unlock instead
> > of retrying directly.
>
> __ceph_remove_cap() must be called even if opening target session
> failed. I think adding a mutex_lock(&session->s_mutex) to the
> IS_ERR(tsession) block should be enough.

YesïI will send the V3 patch later.

> > Thanks.
> > Wu Bo
> >
> > > > +    }
> > > > +
> > > > +    if (mds > target) {
> > > > +            mutex_lock(&session->s_mutex);
> > > > +            mutex_lock_nested(&tsession->s_mutex,
> > > > +                                    SINGLE_DEPTH_NESTING);
> > > > +    } else {
> > > > +            mutex_lock(&tsession->s_mutex);
> > > > +            mutex_lock_nested(&session->s_mutex,
> > > > +                                    SINGLE_DEPTH_NESTING);
> > > >       }
> > > > +    new_cap = ceph_get_cap(mdsc, NULL);
> > > >       goto retry;
> > > >
> > > >    out_unlock:
>
> .