linux-kernel - Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1]

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Date:	Tue, 26 Sep 2006 22:08:56 -0700
From:	Bill Huey (hui) <billh@...ppy.monkey.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"Bill Huey (hui)" <billh@...ppy.monkey.org>
Subject: Re: [PATCH] move put_task_struct() reaping into a thread [Re: 2.6.18-rt1]

On Tue, Sep 26, 2006 at 08:55:41PM -0600, Eric W. Biederman wrote:
> Bill Huey (hui) <billh@...ppy.monkey.org> writes:
> > This patch moves put_task_struct() reaping into a thread instead of an
> > RCU callback function as discussed with Esben publically and Ingo privately:
> 
> Stupid question.

It's a great question actually.

> Why does the rt tree make all calls to put_task_struct an rcu action?
> We only need the rcu callback from kernel/exit.c

Because the conversion of memory allocation routines like kmalloc and kfree aren't
safely callable within a preempt_disable critical section since they were incompletely
converted in the -rt. It can run into the sleeping in atomic scenario which can result
in a deadlock since those routines use blocking locks internally in the implementation
now as a result of the spinlock_t conversion to blocking locks.

> Nothing else needs those semantics.

Right, blame it on the incomplete conversion of the kmalloc and friends. GFP_ATOMIC is
is kind of meaningless in the -rt tree and it might be a good thing to add something
like GFP_RT_ATOMIC for cases like this to be handled properly and restore that particular
semantic in a more meaningful way.

> I agree that put_task_struct is the most common point so this is unlikely
> to remove your issues with rcu callbacks but it just seems completely backwards
> to increase the number of rcu callbacks in the rt tree.

I'm not sure what mean here, but if you mean that you don't like the RCU API abuse then
I agree with you on that. However, Ingo disagrees and I'm not going to argue it with him.
Although, I'm not going stop you if you do. :)

bill

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