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From: Eric Biggers <ebiggers@kernel.org>
To: Gao Xiang <hsiangkao@aol.com>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Daniel Rosenberg <drosen@google.com>,
	Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: [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

WARNING: multiple messages have this Message-ID (diff)
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


_______________________________________________
Linux-f2fs-devel mailing list
Linux-f2fs-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel

  reply	other threads:[~2020-01-24  5:43 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-24  4:12 [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() Eric Biggers
2020-01-24  4:12 ` [f2fs-dev] " Eric Biggers
2020-01-24  5:04 ` Gao Xiang
2020-01-24  5:04   ` [f2fs-dev] " Gao Xiang via Linux-f2fs-devel
2020-01-24  5:16   ` Eric Biggers
2020-01-24  5:16     ` [f2fs-dev] " Eric Biggers
2020-01-24  5:27     ` Gao Xiang
2020-01-24  5:27       ` [f2fs-dev] " Gao Xiang via Linux-f2fs-devel
2020-01-24  5:53       ` Eric Biggers
2020-01-24  5:53         ` [f2fs-dev] " Eric Biggers
2020-01-24  5:34     ` Gao Xiang
2020-01-24  5:34       ` [f2fs-dev] " Gao Xiang via Linux-f2fs-devel
2020-01-24  5:42       ` Eric Biggers [this message]
2020-01-24  5:42         ` Eric Biggers
2020-01-24  6:15         ` Gao Xiang
2020-01-24  6:15           ` [f2fs-dev] " Gao Xiang via Linux-f2fs-devel
2020-01-24 18:12           ` Eric Biggers
2020-01-24 18:12             ` [f2fs-dev] " Eric Biggers
2020-01-24 18:31             ` Al Viro
2020-01-24 18:31               ` [f2fs-dev] " Al Viro
2020-01-25  3:35 ` Theodore Y. Ts'o
2020-01-25  3:35   ` [f2fs-dev] " Theodore Y. Ts'o

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