linux-kernel - Re: [RFC PATCH][3.0] Tracepoint: dissociate from module mutex

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Date:	Wed, 10 Aug 2011 14:05:22 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [RFC PATCH][3.0] Tracepoint: dissociate from module mutex

On Wed, Aug 10, 2011 at 01:41:01PM -0400, Mathieu Desnoyers wrote:
> Copy the information needed from struct module into a local module list
> held within tracepoint.c from within the module coming/going notifier.
> 
> This vastly simplifies locking of tracepoint registration /
> unregistration, because we don't have to take the module mutex to
> register and unregister tracepoints anymore. Steven Rostedt ran into
> dependency problems related to modules mutex vs kprobes mutex vs ftrace
> mutex vs tracepoint mutex that seems to be hard to fix without removing
> this dependency between tracepoint and module mutex. (note: it should be
> investigated whether kprobes could benefit of being dissociated from the
> modules mutex too.)
> 
> This also fixes module handling of tracepoint list iterators, because it
> was expecting the list to be sorted by pointer address. Given we have
> control on our own list now, it's OK to sort this list which has
> tracepoints as its only purpose. The reason why this sorting is required
> is to handle the fact that seq files (and any read() operation from
> user-space) cannot hold the tracepoint mutex across multiple calls, so
> list entries may vanish between calls. With sorting, the tracepoint
> iterator becomes usable even if the list don't contain the exact item
> pointed to by the iterator anymore.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...e.hu>
> CC: Lai Jiangshan <laijs@...fujitsu.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: tglx@...utronix.de
> ---
>  include/linux/module.h     |   12 ---
>  include/linux/tracepoint.h |   25 +++---
>  kernel/module.c            |   47 ------------
>  kernel/tracepoint.c        |  165 +++++++++++++++++++++++++++++++++++++++------
>  4 files changed, 156 insertions(+), 93 deletions(-)

Hi Mathieu,

This is similar to the approach we have taken in the jump label code -
on module insert/remove we store pointers into module table, so that we
don't require the module_mutex during update time. It has been working
well there, so this design makes sense to me at least.

Thanks,

-Jason
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