linux-kernel - Re: [PATCH] markers: comment marker_synchronize

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Message-ID: <20081127080003.GB25072@Krystal>
Date:	Thu, 27 Nov 2008 03:00:03 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Avi Kivity <avi@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH] markers: comment marker_synchronize_unregister() on
	data dependency

* Wu Fengguang (fengguang.wu@...el.com) wrote:
> On Thu, Nov 27, 2008 at 03:23:06AM +0200, Lai Jiangshan wrote:
> > Wu Fengguang wrote:
> > > [updated patch to include Documentation/markers.txt changes]
> > > 
> > > Add document and comments on marker_synchronize_unregister(): it
> > > should be called before freeing resources that the probes depend on.
> > > 
> > > Based on comments from Lai Jiangshan and Mathieu Desnoyers.
> > > 
> > > Cc: Lai Jiangshan <laijs@...fujitsu.com>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> > > Signed-off-by: Wu Fengguang <wfg@...ux.intel.com>
> > > ---
> > > diff --git a/Documentation/markers.txt b/Documentation/markers.txt
> > > index 089f613..8bf6afe 100644
> > > --- a/Documentation/markers.txt
> > > +++ b/Documentation/markers.txt
> > > @@ -51,11 +51,15 @@ to call) for the specific marker through marker_probe_register() and can be
> > >  activated by calling marker_arm(). Marker deactivation can be done by calling
> > >  marker_disarm() as many times as marker_arm() has been called. Removing a probe
> > >  is done through marker_probe_unregister(); it will disarm the probe.
> > > -marker_synchronize_unregister() must be called before the end of the module exit
> > > -function to make sure there is no caller left using the probe. This, and the
> > > -fact that preemption is disabled around the probe call, make sure that probe
> > > -removal and module unload are safe. See the "Probe example" section below for a
> > > -sample probe module.
> > > +
> > > +marker_synchronize_unregister() must be called before the first occurrence of
> > > +- the end of the module exit function,
> > > +  to make sure there is no caller left using the probe;
> > > +- the free of any resource used by the probes,
> > > +  to make sure the probes wont be accessing destructed data.
> > > +This, and the fact that preemption is disabled around the probe call, make sure
> > > +that probe removal and module unload are safe. See the "Probe example" section
> > > +below for a sample probe module.
> > >  
> > >  The marker mechanism supports inserting multiple instances of the same marker.
> > >  Markers can be put in inline functions, inlined static functions, and
> > > diff --git a/include/linux/marker.h b/include/linux/marker.h
> > > index 889196c..32ce4f2 100644
> > > --- a/include/linux/marker.h
> > > +++ b/include/linux/marker.h
> > > @@ -162,8 +162,10 @@ extern void *marker_get_private_data(const char *name, marker_probe_func *probe,
> > >  
> > >  /*
> > >   * marker_synchronize_unregister must be called between the last marker probe
> > > - * unregistration and the end of module exit to make sure there is no caller
> > > - * executing a probe when it is freed.
> > > + * unregistration and the first one of
> > > + * - the end of module exit function
> > > + * - the free of any resource used by the probes
> > 
> > Does "destruction" contain the meaning of "free" and other destruction behavior?
> 
> Ahh, better to use "invalid data" instead of "destructed data"?
> 
> > It's every good job, thank you.
> > 
> > Reviewed-by: Lai Jiangshan <laijs@...fujitsu.com>
> 
> Thank you,
> Fengguang
> ---
> markers: comment marker_synchronize_unregister() on data dependency
> 
> Add document and comments on marker_synchronize_unregister(): it
> should be called before freeing resources that the probes depend on.
> 
> Based on comments from Lai Jiangshan and Mathieu Desnoyers.
> 
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> Reviewed-by: Lai Jiangshan <laijs@...fujitsu.com>
> Signed-off-by: Wu Fengguang <wfg@...ux.intel.com>
> ---
> diff --git a/Documentation/markers.txt b/Documentation/markers.txt
> index 089f613..8bf6afe 100644
> --- a/Documentation/markers.txt
> +++ b/Documentation/markers.txt
> @@ -51,11 +51,15 @@ to call) for the specific marker through marker_probe_register() and can be
>  activated by calling marker_arm(). Marker deactivation can be done by calling
>  marker_disarm() as many times as marker_arm() has been called. Removing a probe
>  is done through marker_probe_unregister(); it will disarm the probe.
> -marker_synchronize_unregister() must be called before the end of the module exit
> -function to make sure there is no caller left using the probe. This, and the
> -fact that preemption is disabled around the probe call, make sure that probe
> -removal and module unload are safe. See the "Probe example" section below for a
> -sample probe module.
> +
> +marker_synchronize_unregister() must be called before the first occurrence of


You should probably say 

".. must be called between probe unregistration and the first occurence
of..."

Mathieu

> +- the end of the module exit function,
> +  to make sure there is no caller left using the probe;
> +- the free of any resource used by the probes,
> +  to make sure the probes wont be accessing invalid data.
> +This, and the fact that preemption is disabled around the probe call, make sure
> +that probe removal and module unload are safe. See the "Probe example" section
> +below for a sample probe module.
>  
>  The marker mechanism supports inserting multiple instances of the same marker.
>  Markers can be put in inline functions, inlined static functions, and
> diff --git a/include/linux/marker.h b/include/linux/marker.h
> index 889196c..32ce4f2 100644
> --- a/include/linux/marker.h
> +++ b/include/linux/marker.h
> @@ -162,8 +162,10 @@ extern void *marker_get_private_data(const char *name, marker_probe_func *probe,
>  
>  /*
>   * marker_synchronize_unregister must be called between the last marker probe
> - * unregistration and the end of module exit to make sure there is no caller
> - * executing a probe when it is freed.
> + * unregistration and the first one of
> + * - the end of module exit function
> + * - the free of any resource used by the probes
> + * to ensure the code and data are valid for any possibly running probes.
>   */
>  #define marker_synchronize_unregister() synchronize_sched()
>  

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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