Re: [PATCH AUTOSEL 5.4 299/350] btrfs: don't prematurely free work in end_workqueue_fn()

[Sasha Levin]

On Thu, Dec 12, 2019 at 01:11:03PM +0100, David Sterba wrote:
> On Tue, Dec 10, 2019 at 04:06:44PM -0500, Sasha Levin wrote:
> > From: Omar Sandoval <osandov@xxxxxx>
> >
> > [ Upstream commit 9be490f1e15c34193b1aae17da58e14dd9f55a95 ]
> >
> > Currently, end_workqueue_fn() frees the end_io_wq entry (which embeds
> > the work item) and then calls bio_endio(). This is another potential
> > instance of the bug in "btrfs: don't prematurely free work in
> > run_ordered_work()".
> >
> > In particular, the endio call may depend on other work items. For
> > example, btrfs_end_dio_bio() can call btrfs_subio_endio_read() ->
> > __btrfs_correct_data_nocsum() -> dio_read_error() ->
> > submit_dio_repair_bio(), which submits a bio that is also completed
> > through a end_workqueue_fn() work item. However,
> > __btrfs_correct_data_nocsum() waits for the newly submitted bio to
> > complete, thus it depends on another work item.
> >
> > This example currently usually works because we use different workqueue
> > helper functions for BTRFS_WQ_ENDIO_DATA and BTRFS_WQ_ENDIO_DIO_REPAIR.
> > However, it may deadlock with stacked filesystems and is fragile
> > overall. The proper fix is to free the work item at the very end of the
> > work function, so let's do that.
> >
> > Reviewed-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
> > Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> > Reviewed-by: David Sterba <dsterba@xxxxxxxx>
> > Signed-off-by: David Sterba <dsterba@xxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>
> The were more patches in the series, all contain "don't prematurely free
> work in" and were part of a rework of async work processing. They're
> fixing a very uncommon usecase, so if there's desire to backport them
> the whole series needs to go in.
>
> In the autosel list, there are only 2 and without the important fix.
>
> c495dcd6fbe1 btrfs: don't prematurely free work in run_ordered_work()
> 9be490f1e15c btrfs: don't prematurely free work in end_workqueue_fn()
> e732fe95e4ca btrfs: don't prematurely free work in reada_start_machine_worker()
> 57d4f0b86327 btrfs: don't prematurely free work in scrub_missing_raid56_worker()

I've queued all 4, thanks!

> a0cac0ec961f btrfs: get rid of unique workqueue helper functions
> - this is only a cleanup that removes code obsoleted by the fixes above,
>  probably out of scope of stable
>
> I have intentionally not tagged the patches for stable, the usecase is
> is specific to one user (FB), the known reproducer is only their
> workload and the fixes are in their kernel already.
>
> So if there's desire to add the patches to stable trees, then it has to
> be the whole series, but I don't see a strong reason for it.

If it's upstream and broken then it's relevant, it doesn't matter if its
one user or a million users.

--
Thanks,
Sasha