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Date: Thu, 23 Jan 2020 21:42:56 -0800 From: Eric Biggers <ebiggers@...nel.org> To: Gao Xiang <hsiangkao@....com> Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, Alexander Viro <viro@...iv.linux.org.uk>, Daniel Rosenberg <drosen@...gle.com>, Gabriel Krisman Bertazi <krisman@...labora.com> Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash() 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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