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

[Justin He]

Hi Al

> -----Original Message-----
> From: Al Viro <viro@xxxxxxxxxxxxxxxx> On Behalf Of Al Viro
> Sent: Monday, June 28, 2021 12:14 PM
> To: Justin He <Justin.He@xxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>; Petr Mladek
> <pmladek@xxxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Sergey
> Senozhatsky <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx>; Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; Heiko
> Carstens <hca@xxxxxxxxxxxxx>; Vasily Gorbik <gor@xxxxxxxxxxxxx>; Christian
> Borntraeger <borntraeger@xxxxxxxxxx>; Eric W . Biederman
> <ebiederm@xxxxxxxxxxxx>; Darrick J. Wong <darrick.wong@xxxxxxxxxx>; Peter
> Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>; Ira Weiny <ira.weiny@xxxxxxxxx>;
> Eric Biggers <ebiggers@xxxxxxxxxx>; Ahmed S. Darwish
> <a.darwish@xxxxxxxxxxxxx>; open list:DOCUMENTATION <linux-
> doc@xxxxxxxxxxxxxxx>; Linux Kernel Mailing List <linux-
> kernel@xxxxxxxxxxxxxxx>; linux-s390 <linux-s390@xxxxxxxxxxxxxxx>; linux-
> fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>
> 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...
IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.