From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH] perf: Use css_tryget to avoid propping up css refcount.
Date: Fri, 15 Jun 2012 10:40:07 +0900
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To: Salman Qazi <sqazi-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

On Thu, Jun 14, 2012 at 03:31:09PM -0700, Salman Qazi wrote:
> An rmdir pushes css's ref count to zero.  However, if the associated
> directory is open at the time, the dentry ref count is non-zero.  If
> the fd for this directory is then passed into perf_event_open, it
> does a css_get().  This bounces the ref count back up from zero.  This
> is a problem by itself.  But what makes it turn into a crash is the
> fact that we end up doing an extra dput, since we perform a dput
> when css_put sees the ref count go down to zero.
> 
> css_tryget does not fall into that trap. So, we use that instead.
> 
...
> Signed-off-by: Salman Qazi <sqazi-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Acked-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>

Peter, can you please pick up this one?

Thanks.

-- 
tejun