linux-kernel - RE: [PATCH 13/14] d_path: prepend_path() is unlikely to return non-zero

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Message-ID: <AM6PR08MB437631C5AD4004EF6B7ADFA1F7039@AM6PR08MB4376.eurprd08.prod.outlook.com>
Date:   Mon, 28 Jun 2021 04:36:24 +0000
From:   Justin He <Justin.He@....com>
To:     Al Viro <viro@...iv.linux.org.uk>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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 13/14] d_path: prepend_path() is unlikely to return
 non-zero

Hi Al

> -----Original Message-----
> From: Al Viro <viro@....linux.org.uk> On Behalf Of Al Viro
> Sent: Monday, June 28, 2021 12:14 PM
> To: Justin He <Justin.He@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>; 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>; 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 13/14] d_path: prepend_path() is unlikely to return
> non-zero
>
> On Mon, Jun 28, 2021 at 03:28:19AM +0000, Justin He wrote:
>
> > > On which loads?  1 here is "mount/dentry pair is in somebody
> > > else's namespace or outside of the subtree we are chrooted
> > > into".  IOW, what's calling d_path() on your setup?
> >
> > No special loads, merely collecting the results after kernel boots up.
> >
> > To narrow down, I tested your branch [1] *without* my '%pD' series:
> > [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/log/?h=work.d_
> path
> >
> > The result is the same after kernel boots up.
>
> IOW, you get 1 in call from d_absolute_path().  And the same commit has
> -       if (prepend_path(path, &root, &b) > 1)
> +       if (unlikely(prepend_path(path, &root, &b) > 1))
> there.  What's the problem?
>
Ah, sorry for the mistake.


--
Cheers,
Justin (Jia He)

> If you want to check mispredictions, put printks at the statements
> under those if (unlikely(...)) and see how often do they trigger...
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