linux-ext4 - Re: [syzbot] KCSAN: data-race in start_this_handle / start_this

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Message-ID: <9dd08907-654c-bc38-fd9f-4324304152af@i-love.sakura.ne.jp>
Date:   Fri, 19 Mar 2021 23:15:42 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Marco Elver <elver@...gle.com>, "Theodore Ts'o" <tytso@....edu>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+30774a6acf6a2cf6d535@...kaller.appspotmail.com>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [syzbot] KCSAN: data-race in start_this_handle /
 start_this_handle

On 2021/03/12 0:54, Marco Elver wrote:
>> But the more we could have the compiler automatically figure out
>> things without needing an explicit tag, it would seem to me that this
>> would be better, since manual tagging is going to be more error-prone.
> 
> What you're alluding to here would go much further than a data race
> detector ("data race" is still just defined by the memory model). The
> wish that there was a static analysis tool that would automatically
> understand the "concurrency semantics as intended by the developer" is
> something that'd be nice to have, but just doesn't seem realistic.
> Because how can a tool tell what the developer intended, without input
> from that developer?

Input from developers is very important for not only compilers and tools
but also allowing bug-explorers to understand what is happening.
ext4 currently has

  possible deadlock in start_this_handle (2)
  https://syzkaller.appspot.com/bug?id=38c060d5757cbc13fdffd46e80557c645fbe79ba

which even maintainers cannot understand what is happening.
How can bug-explorers know implicit logic which maintainers believe safe and correct?
It is possible that some oversight in implicit logic is the cause of
"possible deadlock in start_this_handle (2)".
Making implicit assumptions clear helps understanding.

Will "KCSAN: data-race in start_this_handle / start_this_handle" be addressed by marking?
syzbot is already waiting for
"KCSAN: data-race in jbd2_journal_dirty_metadata / jbd2_journal_dirty_metadata" at
https://syzkaller.appspot.com/bug?id=5eb10023f53097f003e72c6a7c1a6f14b7c22929 .

> 
> If there's worry marking accesses is error-prone, then that might be a
> signal that the concurrency design is too complex (or the developer
> hasn't considered all cases).
> 
> For that reason, we need to mark accesses to tell the compiler and
> tooling where to expect concurrency, so that 1) the compiler generates
> correct code, and 2) tooling such as KCSAN can double-check what the
> developer intended is actually what's happening.

and 3) bug-explorers can understand what the developers are assuming/missing.