Re: [RFCv2 6/7] dlm: use FL_SLEEP to check if blocking request

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2023-08-16 at 21:19 -0400, Alexander Aring wrote:
> Hi,
> 
> On Wed, Aug 16, 2023 at 9:07 AM Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Mon, 2023-08-14 at 17:11 -0400, Alexander Aring wrote:
> > > This patch uses the FL_SLEEP flag in struct file_lock to check if it's a
> > > blocking request in case if the request coming from nfs lockd process
> > > indicated by lm_grant() is set.
> > > 
> > > IF FL_SLEEP is set a asynchronous blocking request is being made and
> > > it's waiting for lm_grant() callback being called to signal the lock was
> > > granted. If it's not set a synchronous non-blocking request is being made.
> > > 
> > > Signed-off-by: Alexander Aring <aahringo@redhat.com>
> > > ---
> > >  fs/dlm/plock.c | 38 ++++++++++++++++++++++----------------
> > >  1 file changed, 22 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/fs/dlm/plock.c b/fs/dlm/plock.c
> > > index 0094fa4004cc..524771002a2f 100644
> > > --- a/fs/dlm/plock.c
> > > +++ b/fs/dlm/plock.c
> > > @@ -140,7 +140,6 @@ int dlm_posix_lock(dlm_lockspace_t *lockspace, u64 number, struct file *file,
> > >       op->info.optype         = DLM_PLOCK_OP_LOCK;
> > >       op->info.pid            = fl->fl_pid;
> > >       op->info.ex             = (fl->fl_type == F_WRLCK);
> > > -     op->info.wait           = IS_SETLKW(cmd);
> > >       op->info.fsid           = ls->ls_global_id;
> > >       op->info.number         = number;
> > >       op->info.start          = fl->fl_start;
> > > @@ -148,24 +147,31 @@ int dlm_posix_lock(dlm_lockspace_t *lockspace, u64 number, struct file *file,
> > >       op->info.owner = (__u64)(long)fl->fl_owner;
> > >       /* async handling */
> > >       if (fl->fl_lmops && fl->fl_lmops->lm_grant) {
> > > -             op_data = kzalloc(sizeof(*op_data), GFP_NOFS);
> > > -             if (!op_data) {
> > > -                     dlm_release_plock_op(op);
> > > -                     rv = -ENOMEM;
> > > -                     goto out;
> > > -             }
> > > +             if (fl->fl_flags & FL_SLEEP) {
> > > +                     op_data = kzalloc(sizeof(*op_data), GFP_NOFS);
> > > +                     if (!op_data) {
> > > +                             dlm_release_plock_op(op);
> > > +                             rv = -ENOMEM;
> > > +                             goto out;
> > > +                     }
> > > 
> > > -             op_data->callback = fl->fl_lmops->lm_grant;
> > > -             locks_init_lock(&op_data->flc);
> > > -             locks_copy_lock(&op_data->flc, fl);
> > > -             op_data->fl             = fl;
> > > -             op_data->file   = file;
> > > +                     op->info.wait = 1;
> > > +                     op_data->callback = fl->fl_lmops->lm_grant;
> > > +                     locks_init_lock(&op_data->flc);
> > > +                     locks_copy_lock(&op_data->flc, fl);
> > > +                     op_data->fl             = fl;
> > > +                     op_data->file   = file;
> > > 
> > > -             op->data = op_data;
> > > +                     op->data = op_data;
> > > 
> > > -             send_op(op);
> > > -             rv = FILE_LOCK_DEFERRED;
> > > -             goto out;
> > > +                     send_op(op);
> > > +                     rv = FILE_LOCK_DEFERRED;
> > > +                     goto out;
> > 
> > A question...we're returning FILE_LOCK_DEFERRED after the DLM request is
> > sent. If it ends up being blocked, what happens? Does it do a lm_grant
> > downcall with -EAGAIN or something as the result?
> > 
> 
> no, when info->wait is set then it is a blocked lock request, which
> means lm_grant() will be called when the lock request is granted.
> 

Ok, that's probably problematic with the current code too. lockd will
time out the block after 7s, so if the lock isn't granted in that time
it'll give up on it.

> > 
> > > +             } else {
> > > +                     op->info.wait = 0;
> > > +             }
> > > +     } else {
> > > +             op->info.wait = IS_SETLKW(cmd);
> > >       }
> > > 
> > >       send_op(op);
> > 
> > Looks reasonable overall.
> > 
> > Now that I look, we have quite a number of places in the kernel that
> > seem to check for F_SETLKW, when what they really want is to check
> > FL_SLEEP.
> 
> Yes, so far I understand FL_SLEEP is F_SETLKW when you get only
> F_SETLK in case of fl->fl_lmops && fl->fl_lmops->lm_grant is true. It
> is confusing but this is how it works... if it's not set we will get
> F_SETLKW and this should imply FL_SLEEP is set.
> 
> 

Yeah. Might be good to consider how to make this more consistent across
the kernel.
-- 
Jeff Layton <jlayton@kernel.org>