Date: Fri, 4 Jan 2008 17:39:05 +0530
From: Gautham R Shenoy <ego@in.ibm.com>
To: Paul E McKenney <paulmck@us.ibm.com>, Steven Rostedt <rostedt@goodmis.org>
Cc: "K. Prasad" <prasad@linux.vnet.ibm.com>, mingo@elte.hu,
       linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
       dipankar@in.ibm.com, Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Subject: __get_cpu_var() called from a preempt-unsafe context in
	__rcu_preempt_unboost() ?
Message-ID: <20080104120905.GA15640@in.ibm.com>
Reply-To: ego@in.ibm.com
References: <20071231060911.GB6461@in.ibm.com> <20080103163055.GB27651@Krystal>
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Hi Paul, Steve,  

This is with reference to the preemptible rcu implementation in
2.6.24-rc5-rt1.

In __rcu_read_unlock(), we call __rcu_preempt_unboost() to unboost a
task's priority which had been bumped up when it was preempted out
while the rcu_read section.

The code path is as follows

__rcu_read_unlock()
	|
	|--> local_irq_restore(oldirq);
	|
	|--> __rcu_preempt_unboost();
		|
		|--> rcu_trace_boost_unboost_called(RCU_BOOST_ME)


where RCU_BOOST_ME is #defined as &__get_cpu_var(rcu_boost_data).

Is calling __get_cpu_var() safe in this context, since we've already
enabled the local interrupts and we're not in a preempt_disabled() ?

Thanks and Regards
gautham.

-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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