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

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Date:	Mon, 29 Sep 2008 23:38:36 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Lai Jiangshan <laijs@...fujitsu.com>
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

* Lai Jiangshan (laijs@...fujitsu.com) wrote:
> 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().
> 

Hum, yes it does separate unregistration and synchronization in two
steps for a very precise purpose : I don't want unregistration of 100
markers to take ~30 seconds on a heavily loaded machine.

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

free(old) is only done in call_rcu. the rcu callback is forced by a
rcu_barrier() if two consecutive operations are done on the same
marker.

>    you fixes haven't fix the reenter bug.
> 

I don't see any reentrancy bug here. Have you actually experienced such
an issue ? Can you give me an example of a bogus execution trace
(step-by-step operations) ?

Thanks,

Mathieu

> 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.
> > 
> 
> 

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