From 8064d54929f23613e649dc7e14f7a94454487d58 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 15 Aug 2009 09:53:49 -0700 Subject: [PATCH] rcu: Make preemptable RCU scan all CPUs when summing RCU counters This patch eliminates the counter-moving during CPU-offline notifiers, eliminating potential confusion if counters are scanned during counter-movement process. This confusion could result in premature ending of an RCU grace period. For example, if there are two tasks in RCU read-side critical sections (so that the sum of the counters is two), and the counter for the CPU going offline is -2, then moving the count to another CPU can result in the sum momentarily appearing to be zero. Since there are no memory barriers in either case, many more such scenarios are possible. So just don't move the counts!!! Signed-off-by: Paul E. McKenney Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: josht@linux.vnet.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: hugh.dickins@tiscali.co.uk Cc: benh@kernel.crashing.org LKML-Reference: <12503552312863-git-send-email-> Signed-off-by: Ingo Molnar --- kernel/rcupreempt.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-) diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c index 9b87f5134ed7..2748b89910b6 100644 --- a/kernel/rcupreempt.c +++ b/kernel/rcupreempt.c @@ -849,7 +849,7 @@ rcu_try_flip_waitzero(void) /* Check to see if the sum of the "last" counters is zero. */ RCU_TRACE_ME(rcupreempt_trace_try_flip_z1); - for_each_cpu(cpu, to_cpumask(rcu_cpu_online_map)) + for_each_possible_cpu(cpu) sum += RCU_DATA_CPU(cpu)->rcu_flipctr[lastidx]; if (sum != 0) { RCU_TRACE_ME(rcupreempt_trace_try_flip_ze1); @@ -1067,12 +1067,6 @@ void rcu_offline_cpu(int cpu) /* seen -after- acknowledgement. */ } - RCU_DATA_ME()->rcu_flipctr[0] += RCU_DATA_CPU(cpu)->rcu_flipctr[0]; - RCU_DATA_ME()->rcu_flipctr[1] += RCU_DATA_CPU(cpu)->rcu_flipctr[1]; - - RCU_DATA_CPU(cpu)->rcu_flipctr[0] = 0; - RCU_DATA_CPU(cpu)->rcu_flipctr[1] = 0; - cpumask_clear_cpu(cpu, to_cpumask(rcu_cpu_online_map)); spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags); -- 2.39.5