linux-ext4 - Re: [PATCH] ext4: fix i_data_sem unlock order in ext4_ind

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Date: Thu, 4 Apr 2024 22:26:51 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: Mikhail Ukhin <mish.uxin2012@...dex.ru>
Cc: stable@...r.kernel.org, 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 i_data_sem unlock order in ext4_ind_migrate()

On Thu, Apr 04, 2024 at 12:50:00PM +0300, Mikhail Ukhin wrote:
> Fuzzing reports a possible deadlock in jbd2_log_wait_commit.
> 
> 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.
> 
> Found by Linux Verification Center (linuxtesting.org) with syzkaller.

This doesn't make any sense.  Lock order matters; the order in which
you unlock shouldn't (and doesn't) make a difference.  This is also
something which lockdep doesn't complain about --- because it's not a
problem.

So how was this "found by syzkaller"?

					- Ted