linux-kernel - Re: [PATCH] ext4/jbd2: drop jbd2_transaction

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Date: Mon, 20 May 2024 10:49:06 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	tytso@....edu, adilger.kernel@...ger.ca, ritesh.list@...il.com,
	yi.zhang@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH] ext4/jbd2: drop jbd2_transaction_committed()

On Thu 16-05-24 16:27:25, Zhang Yi wrote:
> On 2024/5/15 8:25, Jan Kara wrote:
> > On Mon 13-05-24 15:21:19, Zhang Yi wrote:
> > Also accessing j_commit_sequence without any
> > lock is theoretically problematic wrt compiler optimization. You should have
> > READ_ONCE() there and the places modifying j_commit_sequence need to use
> > WRITE_ONCE().
> > 
> 
> Thanks for pointing this out, but I'm not sure if we have to need READ_ONCE()
> here. IIUC, if we add READ_ONCE(), we could make sure to get the latest
> j_commit_sequence, if not, there is a window (it might becomes larger) that
> we could get the old value and jbd2_transaction_committed() could return false
> even if the given transaction was just committed, but I think the window is
> always there, so it looks like it is not a big problem, is that right?

Well, all accesses to any memory should use READ_ONCE(), be protected by a
lock, or use types that handle atomicity on assembly level (like atomic_t,
or atomic bit operations and similar). Otherwise the compiler is free to
assume the underlying memory cannot change and generate potentionally
invalid code. In this case, I don't think realistically any compiler will
do it but still it is a good practice and also it saves us from KCSAN
warnings. If you want to know more details about possible problems, see

  tools/memory-model/Documentation/explanation.txt

chapter "PLAIN ACCESSES AND DATA RACES".

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR