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Date:   Thu, 4 Apr 2019 20:19:08 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Dmitry Vyukov <dvyukov@google.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH v2] acct: fix possible deadlock in acct_pin_kill
Message-ID: <20190404191908.GF2217@ZenIV.linux.org.uk>
References: <20190404105255.12189-1-amir73il@gmail.com>
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On Thu, Apr 04, 2019 at 01:52:55PM +0300, Amir Goldstein wrote:

> If new file is on the same fs as old file, acct_pin_kill(old) fail to
> file_start_write_trylock() and skip writing the old file, because
> sb_writers (of new) is already taken by acct_on().

	The above is BS, BTW.  sb_start_write() does *not*
make file_start_write_trylock() to fail.  It's basically
percpu_down_read() vs. percpu_down_read_trylock().

sb_wait_write() would have file_start_write_trylock() fail (as it
should - its caller is freeze_super(); we want an exclusion with
attempts to start extra writes there).  sb_start_write() sure as
hell doesn't - if it would have, we would get its failures from
things like e.g. truncate(2) somewhere on the same fs.

We don't want to mess with anything freeze-related in acct_on(), but
the bug you are refering to in this part really doesn't exist.