On Fri, Jan 24, 2020 at 02:15:31PM +0800, Gao Xiang wrote: > On Thu, Jan 23, 2020 at 09:42:56PM -0800, Eric Biggers wrote: > > 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 > > After scanning the patch, it seems the parent variable (dentry->d_parent) > only referenced once as below: > > - struct inode *inode = dentry->d_parent->d_inode; > + const struct dentry *parent = READ_ONCE(dentry->d_parent); > + const struct inode *inode = READ_ONCE(parent->d_inode); > > So I think it is enough as > > const struct inode *inode = READ_ONCE(dentry->d_parent->d_inode); > > to access parent inode once to avoid parent inode being accessed > for more time (and all pointers dereference should be in atomic > by compilers) as one reason on > > if (!inode || !IS_CASEFOLDED(inode) || ... > > or etc. > > Thanks for your web reference, I will look into it. I think there > is no worry about dentry->d_parent here because of this only one > dereference on dentry->d_parent. > > You could ignore my words anyway, just my little thought though. > Other part of the patch is ok. > While that does make it really unlikely to cause a real-world problem, it's still undefined behavior to not properly annotate a data race, it would make the code harder to understand as there would be no indication that there's a data race, and it would confuse tools that try to automatically detect data races. So let's keep the READ_ONCE() on d_parent. - Eric
