Re: [PATCH v2] pack-objects: Use packing_data lock instead of read_mutex

[Elijah Newren <newren@gmail.com>]

On Mon, Jan 21, 2019 at 2:02 AM Duy Nguyen <pclouds@gmail.com> wrote:
>
> On Sat, Jan 19, 2019 at 10:45 PM Patrick Hogg <phogg@novamoon.net> wrote:
> >
> > ac77d0c37 ("pack-objects: shrink size field in struct object_entry",
> > 2018-04-14) added an extra usage of read_lock/read_unlock in the newly
> > introduced oe_get_size_slow for thread safety in parallel calls to
> > try_delta(). Unfortunately oe_get_size_slow is also used in serial
> > code, some of which is called before the first invocation of
> > ll_find_deltas. As such the read mutex is not guaranteed to be
> > initialized.
> >
> > Resolve this by using the existing lock in packing_data which is
> > initialized early in cmd_pack_objects instead of read_mutex.
> > Additionally, upgrade the packing_data lock to a recursive mutex to
> > make it a suitable replacement for read_mutex.
> >
> > Signed-off-by: Patrick Hogg <phogg@novamoon.net>
> > ---
> >
> > As I mentioned in the prior thread I think that it will be simpler
> > to simply use the existing lock in packing_data instead of moving
> > read_mutex. I can go back to simply moving read_mutex to the
> > packing_data struct if that that is preferable, though.
>
> In early iterations of these changes, I think we hit high contention
> when sharing the mutex [1]. I don't know if we will hit the same
> performance problem again with this patch. It would be great if Elijah
> with his zillion core machine could test this out. Otherwise it may be
> just safer to keep the two mutexes separate.

Testing...