Re: [PATCH v3 2/6] ext4: for committing inode, make ext4_fc_track_inode wait

[harshad shirwadkar <harshadshirwadkar@gmail.com>]

Thanks for the review. Some questions / comments below:

On Wed, 27 Apr 2022 at 08:50, Jan Kara <jack@suse.cz> wrote:
>
> On Tue 19-04-22 10:31:39, Harshad Shirwadkar wrote:
> > From: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> >
> > If the inode that's being requested to track using ext4_fc_track_inode
> > is being committed, then wait until the inode finishes the
> > commit. Also, add calls to ext4_fc_track_inode at the right places.
> >
> > With this patch, now calling ext4_reserve_inode_write() results in
> > inode being tracked for next fast commit. A subtle lock ordering
> > requirement with i_data_sem (which is documented in the code) requires
> > that ext4_fc_track_inode() be called before grabbing i_data_sem. So,
> > this patch also adds explicit ext4_fc_track_inode() calls in places
> > where i_data_sem grabbed.
> >
> > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> > ---
> >  fs/ext4/fast_commit.c | 38 ++++++++++++++++++++++++++++++++++++++
> >  fs/ext4/inline.c      |  3 +++
> >  fs/ext4/inode.c       |  5 ++++-
> >  3 files changed, 45 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> > index c278060a15bc..55f4c5ddd8e5 100644
> > --- a/fs/ext4/fast_commit.c
> > +++ b/fs/ext4/fast_commit.c
> > +     /*
> > +      * If we come here, we may sleep while waiting for the inode to
> > +      * commit. We shouldn't be holding i_data_sem in write mode when we go
> > +      * to sleep since the commit path needs to grab the lock while
> > +      * committing the inode.
> > +      */
> > +     WARN_ON(lockdep_is_held_type(&ei->i_data_sem, 1));
>
> Note that we can deadlock even if we had i_data_sem for reading because
> another reader is not allowed to get the rwsem if there is writer waiting
> for it. So we need to check even that case here.
I turned the above WARN_ON to check if data_sem is held in either read
or write mode and now I am seeing many other places where data_sem
gets grabbed in read mode before calling ext4_fc_track_inode(). We
either need to call ext4_fc_track_inode() before all
ext4_reserve_inode_write() in all those cases, or ensure that places
that acquire in data_sem in write mode, wait if there's an ongoing
commit and only then lock data_sem.
>
> > +     while (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING)) {
> > +#if (BITS_PER_LONG < 64)
> > +             DEFINE_WAIT_BIT(wait, &ei->i_state_flags,
> > +                             EXT4_STATE_FC_COMMITTING);
> > +             wq = bit_waitqueue(&ei->i_state_flags,
> > +                                EXT4_STATE_FC_COMMITTING);
> > +#else
> > +             DEFINE_WAIT_BIT(wait, &ei->i_flags,
> > +                             EXT4_STATE_FC_COMMITTING);
> > +             wq = bit_waitqueue(&ei->i_flags,
> > +                                EXT4_STATE_FC_COMMITTING);
> > +#endif
> > +             prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
> > +             if (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING))
> > +                     schedule();
> > +             finish_wait(wq, &wait.wq_entry);
> > +     }
> > +
> >       ret = ext4_fc_track_template(handle, inode, __track_inode, NULL, 1);
> >       trace_ext4_fc_track_inode(handle, inode, ret);
>
> As we discussed in the call we should tell lockdep that this is equivalent
> to lock+unlock of let's say fc_committing_lock and the fastcommit code
> setting / clearing EXT4_STATE_FC_COMMITTING is equivalent to lock / unlock
> of fc_committing_lock. That way we get proper lockdep tracking of this
> waiting primitive.
Sure, so you mean just adding __acquires() / __releases() annotations
in these places right?

- Harshad
>
>                                                                 Honza
>
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR