linux-kernel - Re: [PATCH -v4] generic-ipi: remove kmalloc()

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Date:	Tue, 17 Feb 2009 21:02:47 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -v4] generic-ipi: remove kmalloc()

On 02/17, Peter Zijlstra wrote:
>
> +	list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
> +		void (*func)(void *);
> +		void *info;
> +		int refs, wait;
>  
>  		spin_lock(&data->lock);
> -		cpumask_clear_cpu(cpu, to_cpumask(data->cpumask_bits));
> +		if (!cpumask_test_cpu(cpu, data->cpumask)) {
> +			spin_unlock(&data->lock);
> +			continue;
> +		}
> +		cpumask_clear_cpu(cpu, data->cpumask);
>  		WARN_ON(data->refs == 0);
> -		data->refs--;
> -		refs = data->refs;
> +		refs = --data->refs;
> +		func = data->csd.func;
> +		info = data->csd.info;
> +		wait = (data->csd.flags & CSD_FLAG_WAIT);
>  		spin_unlock(&data->lock);
>  
> -		if (refs)
> -			continue;
> +		if (!refs) {
> +			spin_lock(&call_function.lock);
> +			list_del_rcu(&data->csd.list);
> +			spin_unlock(&call_function.lock);
> +			csd_unlock(&data->csd);
> +		}
>  
> -		spin_lock(&call_function_lock);
> -		list_del_rcu(&data->csd.list);
> -		spin_unlock(&call_function_lock);
> +		func(info);
>  
> -		if (data->csd.flags & CSD_FLAG_WAIT) {
> -			/*
> -			 * serialize stores to data with the flag clear
> -			 * and wakeup
> -			 */
> -			smp_wmb();
> -			data->csd.flags &= ~CSD_FLAG_WAIT;
> -		}
> -		if (data->csd.flags & CSD_FLAG_ALLOC)
> -			call_rcu(&data->rcu_head, rcu_free_call_data);
> +		if (!refs && wait)
> +			csd_complete(&data->csd);

Argh! Sorry-sorry-sorry. I managed to confused myself and you.

Please return to the previous version. With this patch we do:

		func(info);

		if (!refs && wait)
			csd_complete();

but we can't do csd_complete(), we can not be sure other CPUs
has already passed this function call.

Of course we can add another counter, but I don't think this
worth the trouble.


As for the previous version. I am almost sure I missed something
again, but do we really need both CSD_FLAG_WAIT and CSD_FLAG_LOCK
flags? smp_call_function_many(wait => 1) can just wait for
!CSD_FLAG_LOCK.

Oleg.

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