linux-kernel - Re: [PATCH] markers: fix unregister bug and reenter bug

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Message-ID: <48E18389.3030202@cn.fujitsu.com>
Date:	Tue, 30 Sep 2008 09:40:25 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] markers: fix unregister bug and reenter bug

Mathieu Desnoyers wrote:
> Hi Lai,
> 
> I'll have to nack this fix :
> 
> One fix I already posted makes sure every marker unregister callers call
> synchronize_sched() _at some point_ before module unload. It thus makes
> sure we can do batch unregistration without doing multiple
> synchronize_sched calls.

1) the new API marker_synchronize_unregister() is ugly, it separate one thing
   into two steps.
   user have to remember to call marker_synchronize_unregister() and have
   to know what he can do and what he can't do before
   marker_synchronize_unregister().

2) IMO, synchronous code is better than asynchronous in non-critical-path.
   we need synchronize_sched() for free(old).

   you fixes haven't fix the reenter bug.

I recommend my fix.

> 
> Also, there is no need to do the synchronize_sched with the marker mutex
> held. call_rcu_sched takes care of making sure the previous quiescent
> state is over before calling kfree. This means that when we return from
> the register/unregister functions, there may still be markers "in
> flight" using the old markers. Again, why would it be a problem ?
> 
> Thanks,
> 
> Mathieu
> 
> P.S. : I'll send along the patches I am referring to. Ingo, those should
> probably be merged if they are not in -tip already.
> 


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