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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2009-10-15 12:26:14 -0400
committerIngo Molnar <mingo@elte.hu>2009-10-15 14:33:01 -0400
commit237c80c5c8fb7ec128cf2a756b550dc41ad7eac7 (patch)
tree5d6b3346f2c53cd3f7471001479a8dbd741533a3 /kernel/rcutree.h
parent019129d595caaa5bd0b41d128308da1be6a91869 (diff)
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang
If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/rcutree.h')
-rw-r--r--kernel/rcutree.h6
1 files changed, 3 insertions, 3 deletions
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 599161f309fb..1823c6e20609 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -306,9 +306,9 @@ static void rcu_print_task_stall(struct rcu_node *rnp);
306#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */ 306#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
307static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp); 307static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
308#ifdef CONFIG_HOTPLUG_CPU 308#ifdef CONFIG_HOTPLUG_CPU
309static void rcu_preempt_offline_tasks(struct rcu_state *rsp, 309static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
310 struct rcu_node *rnp, 310 struct rcu_node *rnp,
311 struct rcu_data *rdp); 311 struct rcu_data *rdp);
312static void rcu_preempt_offline_cpu(int cpu); 312static void rcu_preempt_offline_cpu(int cpu);
313#endif /* #ifdef CONFIG_HOTPLUG_CPU */ 313#endif /* #ifdef CONFIG_HOTPLUG_CPU */
314static void rcu_preempt_check_callbacks(int cpu); 314static void rcu_preempt_check_callbacks(int cpu);