linux-ext4 - Re: [PATCH] ext4: fix semaphore unlocking order

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Date: Thu, 04 Apr 2024 21:32:55 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: Mikhail Ukhin <mish.uxin2012@...dex.ru>, Theodore Ts'o <tytso@....edu>
Cc: Mikhail Ukhin <mish.uxin2012@...dex.ru>, Andreas Dilger <adilger.kernel@...ger.ca>, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, Michail Ivanov <iwanov-23@...ru>, Pavel Koshutin <koshutin.pavel@...dex.ru>, Artem Sadovnikov <ancowi69@...il.com>
Subject: Re: [PATCH] ext4: fix semaphore unlocking order

Ritesh Harjani (IBM) <ritesh.list@...il.com> writes:

> Mikhail Ukhin <mish.uxin2012@...dex.ru> writes:
>
>> Fuzzing reports a possible deadlock in jbd2_log_wait_commit.

I think I agree with what Jan hinted to me in the call, that
how can an unlock order mismatch be a deadlock.

But yes, a wrong unlock order can increase the locking times of
thread-2 waiting on lock B; for e.g. if a premption happens between
unlock of lock A & B by thread-1.

So it is always good to fix the unlock order too.

>>
>> The problem occurs in ext4_ind_migrate due to an incorrect order of
>> unlocking of the journal and write semaphores - the order of unlocking
>> must be the reverse of the order of locking.
>>
>
> Maybe we should update the subject msg to:
>       "ext4: "fix i_data_sem unlock order in ext4_ind_migrate()"
>
> and also should add:
>     CC: stable@...r.kernel.org

In that case, I am not really sure, if this requires a cc'd stable.
So, I will leave this upto Ted.

>
>
> I think this should have been fixed in patch [1], but looks like it
> forgot to fix the unlock order.
>
> [1]: https://lore.kernel.org/all/1364801462-13120-1-git-send-email-dmonakhov@openvz.org/
>
>
>> Found by Linux Verification Center (linuxtesting.org) with syzkaller.

It will be good to know what was the test which identified this though?

-ritesh