linux-ext4 - Re: jbd2: fix deadlock while checkpoint thread waits commit thread to finish (backport to 4.14)

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Date:   Thu, 8 Jul 2021 06:45:35 +0300
From:   Ivan Zahariev <famzah@...zah.net>
To:     Theodore Ts'o <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: jbd2: fix deadlock while checkpoint thread waits commit thread to
 finish (backport to 4.14)

Out of thousand machines, one would trigger the problem about every 1 to 
10 days. Some machines trigger the problem much often than others. So I 
can say that we have a way to verify quickly if applying the patch will 
fix this for us.

The most important question is: Is it safe to apply the patch on 
production machines with kernel 4.14?

We can't risk data loss. And I lack the expertise to asses what risks 
this small patch brings.

Best regards.
--Ivan

On 8.7.2021 г. 2:52, Theodore Ts'o wrote:
> On Wed, Jul 07, 2021 at 09:42:25PM +0300, Ivan Zahariev wrote:
>> Hello,
>>
>> We're running Linux kernel 4.14.x and our systems occasionally suffer a bug
>> which is already fixed: https://github.com/torvalds/linux/commit/53cf978457325d8fb2cdecd7981b31a8229e446e
>>
>> This bugfix hasn't been ported to Linux kernels 4.14 or 4.19. The patch
>> applies cleanly. The two files "fs/jbd2/checkpoint.c" and
>> "fs/jbd2/journal.c" seem pretty identical in the affected sections compared
>> to kernel 5.4 where we have this bugfix already applied.
>>
>> Is it on purpose that this bugfix hasn't been ported to 4.14? Is it safe
>> that we backport it manually in our kernel 4.14 builds? Or is the "ext4"
>> system in 4.14 and 5.4 fundamentally different and this would lead to data
>> loss or other problems?
> The commit was over two years ago, so my memory is not going to be
> perfect.  However, Jan had made a comment suggesting the approach in
> this commit because it should be easier to backport into older stble
> kernels[1].
>
>     "Since proper locking change is going to be a bit more involved, can you
>      perhaps fix this deadlock by just dropping j_checkpoint_mutex in
>      log_do_checkpoint() when we are going to wait for transaction commit. I've
>      checked and that should be fine and that is going to be much easier change
>      to backport into stable kernels..."
>
> [1] https://marc.info/?l=linux-ext4&m=154212553014669&w=2
>
> So I suspect it was just that I failed to remember to add a "Cc:
> stable@...nel.org" and so it was never automatically backported into
> 4.14 or 4.19.
>
> Do you have a reliable reproduction which is triggering the deadlock
> on your kernels?  If so, have you tried applying the patch and does it
> make the problem go away for you?
>
> Cheers,
>
> 						- Ted