Re: [PATCH v5 1/4] xfs: Refactor xfs_isilocked()

[Pavel Reichl <preichl@redhat.com>]

On Wed, Feb 19, 2020 at 9:16 PM Eric Sandeen <sandeen@sandeen.net> wrote:
>
>
>
> On 2/19/20 12:40 PM, Christoph Hellwig wrote:
> > On Tue, Feb 18, 2020 at 08:48:21PM -0800, Darrick J. Wong wrote:
> >>>> +static inline bool
> >>>> +__xfs_rwsem_islocked(
> >>>> +  struct rw_semaphore     *rwsem,
> >>>> +  bool                    shared,
> >>>> +  bool                    excl)
> >>>> +{
> >>>> +  bool locked = false;
> >>>> +
> >>>> +  if (!rwsem_is_locked(rwsem))
> >>>> +          return false;
> >>>> +
> >>>> +  if (!debug_locks)
> >>>> +          return true;
> >>>> +
> >>>> +  if (shared)
> >>>> +          locked = lockdep_is_held_type(rwsem, 0);
> >>>> +
> >>>> +  if (excl)
> >>>> +          locked |= lockdep_is_held_type(rwsem, 1);
> >>>> +
> >>>> +  return locked;
> >>>
> >>> This could use some comments explaining the logic, especially why we
> >>> need the shared and excl flags, which seems very confusing given that
> >>> a lock can be held either shared or exclusive, but not neither and not
> >>> both.
> >>
> >> Yes, this predicate should document that callers are allowed to pass in
> >> shared==true and excl==true when the caller wants to assert that either
> >> lock type (shared or excl) of a given lock class (e.g. iolock) are held.
> >
> > Looking at the lockdep_is_held_type implementation, and our existing
> > code for i_rwsem I really don't see the point of the extra shared
> > check.  Something like:
> >
> > static inline bool
> > __xfs_rwsem_islocked(
> >       struct rw_semaphore     *rwsem,
> >       bool                    excl)
> > {
> >       if (rwsem_is_locked(rwsem)) {
> >               if (debug_locks && excl)
> >                       return lockdep_is_held_type(rwsem, 1);
> >               return true;
> >       }
> >
> >       return false;
> > }
> >
> > should be all that we really need.
>
> I think that's a lot more clear.  In addition to the slight confusion over a (true, true)
> set of args, the current proposal also has the extra confusion of what happens if we pass
> (false, false), for example.
>
> One other thought, since debug_locks getting turned off by lockdep means that
> an exclusive test reverts to a shared|exclusive test, would it be worth adding
> a WARN_ON_ONCE to make it clear when xfs rwsem lock testing coverage has been
> reduced?
>
> -Eric
>

OK, thanks for the comments.

Eric in the following code is WARN_ONCE() used as you suggested or did
you have something else in mind?

static inline bool
__xfs_rwsem_islocked(
        struct rw_semaphore     *rwsem,
        bool                    excl)
{
        if (!rwsem_is_locked(rwsem)) {
                return false;
        }

        if (excl) {
                if (debug_locks) {
                        return lockdep_is_held_type(rwsem, 1);
                }
                WARN_ONCE(1,
                        "xfs rwsem lock testing coverage has been reduced\n");
        }
        return true;
}