From: Alexander Aring <aahringo@redhat.com>
To: Andreas Gruenbacher <agruenba@redhat.com>
Cc: gfs2 <gfs2@lists.linux.dev>,
teigland@redhat.com, mark@fasheh.com, jlbec@evilplan.org,
joseph.qi@linux.alibaba.com, ocfs2-devel@lists.linux.dev
Subject: Re: [PATCHv3 v6.5-rc2 2/3] fs: dlm: allow to F_SETLKW getting interrupted
Date: Tue, 26 Mar 2024 09:02:07 -0400 [thread overview]
Message-ID: <CAK-6q+hUwaumWfXHktsxxzDtGHx-JeCGqxBjSBbYPf8bMHB+Ew@mail.gmail.com> (raw)
In-Reply-To: <CAHc6FU6ii3NXm-_eraNTW-UEKEuR5gr+Qi2WeefRxdxH-GJrtw@mail.gmail.com>
Hi,
On Tue, Mar 26, 2024 at 7:31 AM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
> On Tue, Mar 26, 2024 at 1:32 AM Alexander Aring <aahringo@redhat.com> wrote:
> > Hi,
> >
> > On Mon, Mar 25, 2024 at 11:09 AM Andreas Gruenbacher
> > <agruenba@redhat.com> wrote:
> > >
> > > On Tue, Jul 18, 2023 at 8:07 PM Alexander Aring <aahringo@redhat.com> wrote:
> > > > This patch implements dlm plock F_SETLKW interruption feature. If a
> > > > blocking posix lock request got interrupted in user space by a signal a
> > > > cancellation request for a non granted lock request to the user space
> > > > lock manager will be send. The user lock manager answers either with
> > > > zero or a negative errno code. A errno of -ENOENT signals that there is
> > > > currently no blocking lock request waiting to being granted. In case of
> > > > -ENOENT it was probably to late to request a cancellation and the
> > > > pending lock got granted. In any error case we will wait until the lock
> > > > is being granted as cancellation failed, this causes also that in case
> > > > of an older user lock manager returning -EINVAL we will wait as
> > > > cancellation is not supported which should be fine. If a user requires
> > > > this feature the user should update dlm user space to support lock
> > > > request cancellation.
> > > >
> > > > Signed-off-by: Alexander Aring <aahringo@redhat.com>
> > > > ---
> > > > fs/dlm/plock.c | 56 ++++++++++++++++++++++------------
> > > > include/uapi/linux/dlm_plock.h | 1 +
> > > > 2 files changed, 37 insertions(+), 20 deletions(-)
> > > >
> > > > diff --git a/fs/dlm/plock.c b/fs/dlm/plock.c
> > > > index a34f605d8505..a8ffa0760913 100644
> > > > --- a/fs/dlm/plock.c
> > > > +++ b/fs/dlm/plock.c
> > > > @@ -74,30 +74,26 @@ static void send_op(struct plock_op *op)
> > > > wake_up(&send_wq);
> > > > }
> > > >
> > > > -/* If a process was killed while waiting for the only plock on a file,
> > > > - locks_remove_posix will not see any lock on the file so it won't
> > > > - send an unlock-close to us to pass on to userspace to clean up the
> > > > - abandoned waiter. So, we have to insert the unlock-close when the
> > > > - lock call is interrupted. */
> > > > -
> > > > -static void do_unlock_close(const struct dlm_plock_info *info)
> > > > +static int do_lock_cancel(const struct dlm_plock_info *orig_info)
> > > > {
> > > > struct plock_op *op;
> > > > + int rv;
> > > >
> > > > op = kzalloc(sizeof(*op), GFP_NOFS);
> > > > if (!op)
> > > > - return;
> > > > + return -ENOMEM;
> > > > +
> > > > + op->info = *orig_info;
> > > > + op->info.optype = DLM_PLOCK_OP_CANCEL;
> > > > + op->info.wait = 0;
> > > >
> > > > - op->info.optype = DLM_PLOCK_OP_UNLOCK;
> > > > - op->info.pid = info->pid;
> > > > - op->info.fsid = info->fsid;
> > > > - op->info.number = info->number;
> > > > - op->info.start = 0;
> > > > - op->info.end = OFFSET_MAX;
> > > > - op->info.owner = info->owner;
> > > > -
> > > > - op->info.flags |= DLM_PLOCK_FL_CLOSE;
> > > > send_op(op);
> > > > + wait_event(recv_wq, (op->done != 0));
> > > > +
> > > > + rv = op->info.rv;
> > > > +
> > > > + dlm_release_plock_op(op);
> > > > + return rv;
> > > > }
> > > >
> > > > int dlm_posix_lock(dlm_lockspace_t *lockspace, u64 number, struct file *file,
> > > > @@ -156,7 +152,7 @@ int dlm_posix_lock(dlm_lockspace_t *lockspace, u64 number, struct file *file,
> > > > send_op(op);
> > > >
> > > > if (op->info.wait) {
> > > > - rv = wait_event_killable(recv_wq, (op->done != 0));
> > > > + rv = wait_event_interruptible(recv_wq, (op->done != 0));
> > >
> > > It seems that this patch leads to an unnecessary change in behavior
> > > when a fatal signal is received (fatal_signal_pending()): before, the
> > > process would terminate. Now, it will try to cancel the lock, and when
> > > that fails, the process will keep waiting. In case of a fatal signal,
> > > can we skip the cancelling and do what we did before?
> >
> > From my understanding interruptible() "reacts" on everything that is
> > also killable() and returns -ERESTARTSYS on "fatal signal". I even
> > tested it because wait_event_killable() has an issue, see reproducer
> > [0]. The issue was that it cleans too many waiters, the other waiter
> > of child in F_SETLKW was also cleared and it will never get a result
> > back from dlm_controld. I fixed that with an additional check on pid
> > in [1], but I have no idea about other side effects that could have
> > occurred as FL_CLOSE is also being used on other parts in the DLM
> > plock handling.
> >
> > I rechecked the behaviour with the cancellation feature and sent
> > SIGKILL and the issue was gone without changing anything in user
> > space. The only thing I see why it would not have the old behaviour
> > (killable - that having the mentioned issue above) is that the
> > dlm_controld version is too old. To not run into this known issue we
> > just do a wait_event() that does not have those issues.
> >
> > The mentioned "cancellation fails" - is not that it failed to cancel
> > the lock, there is some unexpected behaviour of dlm_controld, only
> > then we do wait_event() e.g. when we receive -EINVAL because
> > dlm_controld does not understand the op.
>
> What happens on a system that has this kernel change, but doesn't have
> the corresponding dlm_controld change for DLM_PLOCK_OP_CANCEL support?
> In this scenario, when a process is killed with SIGKILL, the kernel
> will send a DLM_PLOCK_OP_CANCEL request to dlm_controld. dlm_controld
> doesn't understand the DLM_PLOCK_OP_CANCEL request, so I assume that
> it will return a value other than 0 or -ENOENT. As a consequence, we
> will end up in wait_event(), which isn't interruptible. So before this
> kernel change, the process would be killable, but with this kernel
> change, it isn't killable anymore.
>
> I'm worried about this scenario because it isn't entirely unrealistic
> for a system to end up with this kernel change but without the
> corresponding user-space change.
>
as I said before the previous behaviour had broken cases. I am more
worried about that somebody runs into this case as somebody realize
its not killable anymore but will may then figure out to upgrade
dlm_controld.
- Alex
next prev parent reply other threads:[~2024-03-26 13:02 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-18 18:07 [PATCHv3 v6.5-rc2 0/3] fs: dlm: lock cancellation feature Alexander Aring
2023-07-18 18:07 ` [PATCHv3 v6.5-rc2 1/3] fs: dlm: remove twice newline Alexander Aring
2023-07-18 18:07 ` [PATCHv3 v6.5-rc2 2/3] fs: dlm: allow to F_SETLKW getting interrupted Alexander Aring
2024-03-25 15:08 ` Andreas Gruenbacher
2024-03-26 0:32 ` Alexander Aring
2024-03-26 11:31 ` Andreas Gruenbacher
2024-03-26 13:02 ` Alexander Aring [this message]
2023-07-18 18:07 ` [PATCHv3 v6.5-rc2 3/3] fs: dlm: fix F_CANCELLK to cancel pending request Alexander Aring
2023-07-19 18:33 ` Alexander Aring
2023-07-25 14:26 ` Alexander Aring
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