linux-kernel - Re: [PATCH RFC 1/3] fs: introduce helper d_path

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Date:   Sun, 9 May 2021 02:28:23 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jia He <justin.he@....com>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@....linux.org.uk>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

On Sat, May 08, 2021 at 03:47:38PM -0700, Linus Torvalds wrote:
> On Sat, May 8, 2021 at 3:42 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > 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).
> 
> Oh, and looking at it some more, I think it would probably be a good
> thing to make __dentry_path() take a
> 
>        struct prepend_buffer *orig
> 
> argument, the same way prepend_path() does. Also, like prepend_path(),
> the terminating NUL should probably be done by the caller.
> 
> Doing those two changes would simplify the hackery we now have in
> "dentry_path()" due to the "//deleted" games. The whole "restore '/'
> that was overwritten by the NUL added by __dentry_path() is
> unbelievably ugly.

	Agreed.  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.