From: Zhang Yi <yi.zhang@huaweicloud.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, tytso@mit.edu,
adilger.kernel@dilger.ca, ritesh.list@gmail.com,
yi.zhang@huawei.com, chengzhihao1@huawei.com, yukuai3@huawei.com
Subject: Re: [PATCH] ext4/jbd2: drop jbd2_transaction_committed()
Date: Mon, 20 May 2024 20:39:51 +0800 [thread overview]
Message-ID: <8d85f75e-cf9e-b3da-766f-59d80d608203@huaweicloud.com> (raw)
In-Reply-To: <20240520084906.ejykv3xwn7l36jbg@quack3>
On 2024/5/20 16:49, Jan Kara wrote:
> 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".
>
Sure, this document is really helpful, I'll add READ_ONCE() and
WRITE_ONCE() here, thanks a lot.
Yi.
next prev parent reply other threads:[~2024-05-20 12:39 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-13 7:21 [PATCH] ext4/jbd2: drop jbd2_transaction_committed() Zhang Yi
2024-05-15 0:25 ` Jan Kara
2024-05-16 8:27 ` Zhang Yi
2024-05-20 8:49 ` Jan Kara
2024-05-20 12:39 ` Zhang Yi [this message]
2024-05-27 6:37 ` kernel test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=8d85f75e-cf9e-b3da-766f-59d80d608203@huaweicloud.com \
--to=yi.zhang@huaweicloud.com \
--cc=adilger.kernel@dilger.ca \
--cc=chengzhihao1@huawei.com \
--cc=jack@suse.cz \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=ritesh.list@gmail.com \
--cc=tytso@mit.edu \
--cc=yi.zhang@huawei.com \
--cc=yukuai3@huawei.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).