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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 8 May 2021 15:42:00 -0700
Message-ID: <CAHk-=wjhrhkWbV_EY0gupi2ea7QHpGW=68x7g09j_Tns5ZnsLA@mail.gmail.com>
Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Jia He <justin.he@arm.com>, Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@ftp.linux.org.uk>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Eric Biggers <ebiggers@google.com>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
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On Sat, May 8, 2021 at 2:06 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> FWIW, if we go that way, I would make that
>
>         while (dentry != root->dentry || &mnt->mnt != root->mnt) {
>                 int error;
>                 struct dentry *parent = READ_ONCE(dentry->d_parent);

Side note: you've added that READ_ONCE() to the parent reading, and I
think that's a bug-fix regardless. The old code does that plain

                parent = dentry->d_parent;

(after doing the mountpoint stuff). And d_parent isn't actually
guaranteed stable here.

It probably does not matter - we are in a RCU read-locked section, so
it's not like parent will go away, but in theory we might end up with
(for example) pre-fetching a different parent than the one we then
walk down.

But your READ_ONCE() is definitely the right thing to do (whether we
do your re-org or not, and whether we do this "prepend_buffer" thing
or not).

Do you want to do a final version with your fixes?

               Linus