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

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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-
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> 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...
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