Re: [PATCH] ns: move free_nsproxy() out of do_exit() path

From: Kirill A. Shutemov
Date: Fri Jul 13 2012 - 18:22:50 EST


On Fri, Jul 13, 2012 at 02:08:06PM -0700, Andrew Morton wrote:
> On Fri, 13 Jul 2012 14:48:08 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > free_nsproxy() is too heavy to be on exit path. Let's free namespaces
> > asynchronously to not block exit_group() syscall.
>
> Please be specific, and complete.
>
> Why is it "too heavy"? Where is the time being spent? Is it spent in
> D state or is it spent burning CPU cycles? Does the patch simply
> offload the work into kernel threads, providing no net gain?

Unpatched switch_task_namespaces() takes 0.010 - 0.011 seconds on my
machine. About 0.008 of the time is synchronize_rcu().

So it's mostly waiting with wait_for_completion() in wait_rcu_gp().
It means D state.

> > The patch also fixes bug with free namespace without synchronize_rcu() through
> > put_nsproxy().
>
> I just don't understand this description.

IIUC current locking model requires synchronize_rcu() before
free_nsproxy(). put_nsproxy() calls free_nsproxy() without
synchronize_rcu(). So it's racy.

I guess it was missed during switch to RCU (see cf7b708).
Pavel, am I right?

> Please send a new one which
> includes all details about the bug, including a description of
> the user-visible effects of the bug.

Okay, I will.

--
Kirill A. Shutemov
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