linux-kernel - Re: [PATCH AUTOSEL 5.4 299/350] btrfs: don't prematurely free work in end_workqueue

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Date:   Thu, 19 Dec 2019 18:06:49 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     dsterba@...e.cz, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Omar Sandoval <osandov@...com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.4 299/350] btrfs: don't prematurely free work
 in end_workqueue_fn()

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@...com>
>>
>> [ 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@...e.de>
>> Signed-off-by: Omar Sandoval <osandov@...com>
>> Reviewed-by: David Sterba <dsterba@...e.com>
>> Signed-off-by: David Sterba <dsterba@...e.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>
>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