linux-kernel - Re: __get_cpu_var() called from a preempt-unsafe context in __rcu_preempt

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Date:	Fri, 4 Jan 2008 08:48:03 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Gautham R Shenoy <ego@...ibm.com>
cc:	Paul E McKenney <paulmck@...ibm.com>,
	"K. Prasad" <prasad@...ux.vnet.ibm.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	dipankar@...ibm.com,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: __get_cpu_var() called from a preempt-unsafe context in
 __rcu_preempt_unboost() ?


On Fri, 4 Jan 2008, Gautham R Shenoy wrote:
> 		|
> 		|--> 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() ?

Probably not, but the worst that can happen is that we corrupt the trace
counter, and miss an increment.  I'll see if I can fix that, but it should
be quite harmless wrt the stability of the system.

Thanks for pointing that out.

-- Steve
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