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

[Wu Bo]

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.

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: