From: Eric Biggers <ebiggers@kernel.org>
To: Gao Xiang <hsiangkao@aol.com>
Cc: Daniel Rosenberg <drosen@google.com>,
linux-f2fs-devel@lists.sourceforge.net,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: [f2fs-dev] [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash()
Date: Thu, 23 Jan 2020 21:42:56 -0800 [thread overview]
Message-ID: <20200124054256.GC832@sol.localdomain> (raw)
In-Reply-To: <20200124053415.GC31271@hsiangkao-HP-ZHAN-66-Pro-G1>
On Fri, Jan 24, 2020 at 01:34:23PM +0800, Gao Xiang wrote:
> On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote:
>
> []
>
> > So we need READ_ONCE() to ensure that a consistent value is used.
>
> By the way, my understanding is all pointer could be accessed
> atomicly guaranteed by compiler. In my opinion, we generally
> use READ_ONCE() on pointers for other uses (such as, avoid
> accessing a variable twice due to compiler optimization and
> it will break some logic potentially or need some data
> dependency barrier...)
>
> Thanks,
> Gao Xiang
But that *is* why we need READ_ONCE() here. Without it, there's no guarantee
that the compiler doesn't load the variable twice. Please read:
https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE
- Eric
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next prev parent reply other threads:[~2020-01-24 5:43 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-24 4:12 [f2fs-dev] [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() Eric Biggers
2020-01-24 5:04 ` Gao Xiang via Linux-f2fs-devel
2020-01-24 5:16 ` Eric Biggers
2020-01-24 5:27 ` Gao Xiang via Linux-f2fs-devel
2020-01-24 5:53 ` Eric Biggers
2020-01-24 5:34 ` Gao Xiang via Linux-f2fs-devel
2020-01-24 5:42 ` Eric Biggers [this message]
2020-01-24 6:15 ` Gao Xiang via Linux-f2fs-devel
2020-01-24 18:12 ` Eric Biggers
2020-01-24 18:31 ` Al Viro
2020-01-25 3:35 ` Theodore Y. Ts'o
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