linux-kernel - Re: [PATCH] Remove stop_machine during module load

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Message-ID: <20080829212330.GC26610@one.firstfloor.org>
Date:	Fri, 29 Aug 2008 23:23:30 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: Re: [PATCH] Remove stop_machine during module load

Hi Paul,

Thanks for the excellent review.

On Fri, Aug 29, 2008 at 01:44:57PM -0700, Paul E. McKenney wrote:
> 
> OK, what about the read side?  Not so good for __unlink_module() to yank

That's independent from my patch isn't it? I don't think I'm changing
anything here. All of the issues you're pointing out are already
in the code (except for the missing read_barrier_depends() perhaps)

I think the lockless users like oops or sysrq-t typically have preemption
disabled, so they should be ok regarding that. 

> the module out from under a reader.  Therefore, all readers must either
> disable interrupts to block stop_machine() or must hold some sort of
> mutex that prevents modules from being unloaded.
> 
> First, where the heck -is- the read side...
> 
> o	each_symbol() needs its list_for_each_entry() to become
> 	list_for_each_entry_rcu() and needs local_irq_disable()

Ah that's needed for the Alpha barrier depends semantics,
right? 

> 	Yet another approach would be to use call_rcu() to defer the
> 	various kfree() &c calls later in free_module.

I think that would be a the better approach.

-Andi

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