linux-kernel - Re: [PATCH V2] fs/ceph:fix double unlock in handle_cap

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Date:   Thu, 30 Apr 2020 10:50:57 +0800
From:   "Yan, Zheng" <ukernel@...il.com>
To:     Wu Bo <wubo40@...wei.com>
Cc:     Jeff Layton <jlayton@...nel.org>, Sage Weil <sage@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        liuzhiqiang26@...wei.com, linfeilong@...wei.com
Subject: Re: [PATCH V2] fs/ceph:fix double unlock in handle_cap_export()

On Wed, Apr 29, 2020 at 8:49 AM Wu Bo <wubo40@...wei.com> 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@...wei.com>
> >> ---
> >>   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.


> 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:
> >
>
>