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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 8 May 2021 19:53:26 -0700
Message-ID: <CAHk-=wih_O+0xG4QbLw-3XJ71Yh43_SFm3gp9swj8knzXoceZQ@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 7:28 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
>         Re READ_ONCE() - we are wrapped into
> read_seqbegin_or_lock(&rename_lock, &seq) there, so it's more about
> being explicit than about correctness considerations.

Well, part of this all is that the next step is that "vsnprintf()"
with '%pD' would basically use prepend_entries() with just the RCU
lock.

That said, even with the rename lock, that will only cause a retry on
rename - it won't necessarily fix any confusion that comes from the
compiler possibly silently re-loading 'parent' multiple times, and
getting different pointers due to a concurrent rename.

Now, those different results should all be individually ok, due to RCU
freeing, but it's _really_ confusing if 'parent' might be two
different things within the same iteration of the loop.

I don't see anything truly horrible that would happen - mainly "we'll
prefetch one parent, and then due to reloading the pointer we might
actually _use_ another parent entirely for the next iteration", but it
really is best to avoid that kind of confusion.

                Linus