rcu: Keep gpnum and completed fields synchronized
When a CPU that was in an extended quiescent state wakes up and catches up with grace periods that remote CPUs completed on its behalf, we update the completed field but not the gpnum that keeps a stale value of a backward grace period ID. Later, note_new_gpnum() will interpret the shift between the local CPU and the node grace period ID as some new grace period to handle and will then start to hunt quiescent state. But if every grace periods have already been completed, this interpretation becomes broken. And we'll be stuck in clusters of spurious softirqs because rcu_report_qs_rdp() will make this broken state run into infinite loop. The solution, as suggested by Lai Jiangshan, is to ensure that the gpnum and completed fields are well synchronized when we catch up with completed grace periods on their behalf by other cpus. This way we won't start noting spurious new grace periods. Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Steven Rostedt <rostedt@goodmis.org Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -679,6 +679,15 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
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/* Remember that we saw this grace-period completion. */
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/* Remember that we saw this grace-period completion. */
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rdp->completed = rnp->completed;
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rdp->completed = rnp->completed;
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/*
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* If we were in an extended quiescent state, we may have
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* missed some grace periods that others CPUs took care on
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* our behalf. Catch up with this state to avoid noting
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* spurious new grace periods.
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*/
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if (rdp->completed > rdp->gpnum)
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rdp->gpnum = rdp->completed;
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/*
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/*
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* If another CPU handled our extended quiescent states and
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* If another CPU handled our extended quiescent states and
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* we have no more grace period to complete yet, then stop
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* we have no more grace period to complete yet, then stop
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