Commit c2d727aa authored by Dipankar Sarma's avatar Dipankar Sarma Committed by Ingo Molnar
Browse files

Preempt-RCU: Use softirq instead of tasklets for 

This patch makes RCU use softirq instead of tasklets.

It also adds a memory barrier after raising the softirq
inorder to ensure that the cpu sees the most recently updated
value of rcu->cur while processing callbacks.
The discussion of the related theoretical race pointed out
by James Huang can be found here --> http://lkml.org/lkml/2007/11/20/603



Signed-off-by: default avatarGautham R Shenoy <ego@in.ibm.com>
Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarDipankar Sarma <dipankar@in.ibm.com>
Reviewed-by: default avatarSteven Rostedt <srostedt@redhat.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent c49443c5

include/linux/interrupt.h

+1 −0
Original line number Diff line number Diff line
@@ -256,6 +256,7 @@ enum 
#ifdef CONFIG_HIGH_RES_TIMERS

	HRTIMER_SOFTIRQ,

#endif

	RCU_SOFTIRQ, 	/* Preferable RCU should always be the last softirq */

};



/* softirq mask and active fields moved to irq_cpustat_t in

kernel/rcupdate.c

+17 −8
Original line number Diff line number Diff line
@@ -73,8 +73,6 @@ static struct rcu_ctrlblk rcu_bh_ctrlblk = { 
DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };

DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };



/* Fake initialization required by compiler */

static DEFINE_PER_CPU(struct tasklet_struct, rcu_tasklet) = {NULL};

static int blimit = 10;

static int qhimark = 10000;

static int qlowmark = 100;
@@ -231,6 +229,18 @@ void rcu_barrier(void) 
}

EXPORT_SYMBOL_GPL(rcu_barrier);



/* Raises the softirq for processing rcu_callbacks. */

static inline void raise_rcu_softirq(void)

{

	raise_softirq(RCU_SOFTIRQ);

	/*

	 * The smp_mb() here is required to ensure that this cpu's

	 * __rcu_process_callbacks() reads the most recently updated

	 * value of rcu->cur.

	 */

	smp_mb();

}



/*

 * Invoke the completed RCU callbacks. They are expected to be in

 * a per-cpu list.
@@ -260,7 +270,7 @@ static void rcu_do_batch(struct rcu_data *rdp) 
	if (!rdp->donelist)

		rdp->donetail = &rdp->donelist;

	else

		tasklet_schedule(&per_cpu(rcu_tasklet, rdp->cpu));

		raise_rcu_softirq();

}



/*
@@ -412,7 +422,6 @@ static void rcu_offline_cpu(int cpu) 
					&per_cpu(rcu_bh_data, cpu));

	put_cpu_var(rcu_data);

	put_cpu_var(rcu_bh_data);

	tasklet_kill_immediate(&per_cpu(rcu_tasklet, cpu), cpu);

}



#else
@@ -424,7 +433,7 @@ static void rcu_offline_cpu(int cpu) 
#endif



/*

 * This does the RCU processing work from tasklet context. 

 * This does the RCU processing work from softirq context.

 */

static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,

					struct rcu_data *rdp)
@@ -469,7 +478,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp, 
		rcu_do_batch(rdp);

}



static void rcu_process_callbacks(unsigned long unused)

static void rcu_process_callbacks(struct softirq_action *unused)

{

	__rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));

	__rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
@@ -533,7 +542,7 @@ void rcu_check_callbacks(int cpu, int user) 
		rcu_bh_qsctr_inc(cpu);

	} else if (!in_softirq())

		rcu_bh_qsctr_inc(cpu);

	tasklet_schedule(&per_cpu(rcu_tasklet, cpu));

	raise_rcu_softirq();

}



static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
@@ -556,7 +565,7 @@ static void __cpuinit rcu_online_cpu(int cpu) 


	rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);

	rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);

	tasklet_init(&per_cpu(rcu_tasklet, cpu), rcu_process_callbacks, 0UL);

	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks, NULL);

}



static int __cpuinit rcu_cpu_notify(struct notifier_block *self,

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