From: Al Viro <viro@zeniv.linux.org.uk>
To: Qian Cai <cai@lca.pw>
Cc: hch@infradead.org, darrick.wong@oracle.com, elver@google.com,
linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: fix a data race in i_size_write/i_size_read
Date: Wed, 19 Feb 2020 05:23:29 +0000 [thread overview]
Message-ID: <20200219052329.GP23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <EDCBB5B9-DEE4-4D4B-B2F4-F63179977D6C@lca.pw>
On Wed, Feb 19, 2020 at 12:08:40AM -0500, Qian Cai wrote:
>
>
> > On Feb 18, 2020, at 11:52 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up
> > being split, the kernel is FUBAR anyway. Details, please - how could that
> > end up happening?
>
> My understanding is the compiler might decide to split the load into saying two 4-byte loads. Then, we might have,
>
> Load1
> Store
> Load2
>
> where the load value could be a garbage. Also, Marco (the KCSAN maintainer) who knew more of compiler than me mentioned that there is no guarantee that the store will not be split either. Thus, the WRITE_ONCE().
>
I would suggest
* if some compiler does that, ask the persons responsible for that
"optimization" which flags should be used to disable it.
* if they fail to provide such, educate them regarding the
usefulness of their idea
* if that does not help, don't use the bloody piece of garbage.
Again, is that pure theory (because I can't come up with
any reason why splitting a 32bit load would be any less legitimate
than doing the same to a 64bit one on a 64bit architecture),
or is there anything that really would pull that off?
next prev parent reply other threads:[~2020-02-19 5:23 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-19 4:04 [PATCH] fs: fix a data race in i_size_write/i_size_read Qian Cai
2020-02-19 4:52 ` Al Viro
2020-02-19 5:08 ` Qian Cai
2020-02-19 5:23 ` Al Viro [this message]
2020-02-19 9:21 ` Marco Elver
2020-02-19 12:18 ` David Sterba
2020-02-21 4:19 ` Qian Cai
2020-02-19 12:08 ` David Sterba
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=20200219052329.GP23230@ZenIV.linux.org.uk \
--to=viro@zeniv.linux.org.uk \
--cc=cai@lca.pw \
--cc=darrick.wong@oracle.com \
--cc=elver@google.com \
--cc=hch@infradead.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-xfs@vger.kernel.org \
/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 NNTP newsgroup(s).