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

Justin He
2021-06-28T04:36:43Z

Hi Al

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Monday, June 28, 2021 12:14 PM
> To: Justin He <Justin.He@arm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>; 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>; 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
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> 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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