linux-kernel - Re: Question about usage of RCU in the input layer

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Message-ID: <49C33928.1050806@cosmosbay.com>
Date:	Fri, 20 Mar 2009 07:35:20 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	dipankar@...ibm.com
CC:	paulmck@...ux.vnet.ibm.com, Arjan van de Ven <arjan@...radead.org>,
	linux-input@...r.kernel.org, dmitry.torokhov@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

Dipankar Sarma a écrit :
> On Fri, Mar 20, 2009 at 06:28:11AM +0100, Eric Dumazet wrote:
>> Paul E. McKenney a écrit :
>> Hmm... point is to make linux boot as fast as possible, so ...
>>
>> Use a special variant of udelay() in offending drivers that make appropriate
>> RCU call to increment quiescent state ?
> 
> It may not be safe to do this it is in a RCU read-side critical section.

Yes, this is why I suggested a variant of udelay, not udelay() itself,
used by selected drivers (probably not so many drivers hog cpu so long)

void udelay_rcu_quiescent(unsigned long usecs)
{
	preempt_disable();
	rcu_qsctr_inc(smp_processor_id());
	preempt_enable();

	udelay(usecs);
}


Or maybe we have a way to detect we are in a RCU-side critical section at runtime ?

void __udelay(unsigned long usecs)
{
	if (!in_rcu_critical_section()) {
		preempt_disable();
		rcu_qsctr_inc(smp_processor_id());
		preempt_enable();
	}
	...
}


Thank you

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