[tip:core/rcu] rcu: Delay rcu_barrier() wait until beginning of next CPU-hotunplug operation.

tip-bot for Paul E. McKenney
2009-08-19T12:10:20Z

Commit-ID: 1423cc033df017c762a9155eec470da77a460141
Gitweb: http://git.kernel.org/tip/1423cc033df017c762a9155eec470da77a460141
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
AuthorDate: Tue, 18 Aug 2009 23:06:14 -0700
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 19 Aug 2009 13:58:54 +0200

rcu: Delay rcu_barrier() wait until beginning of next CPU-hotunplug operation.

Ingo Molnar reported this lockup:

[ 200.380003] Hangcheck: hangcheck value past margin!
[ 248.192003] INFO: task S99local:2974 blocked for more than 120 seconds.
[ 248.194532] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 248.202330] S99local D 0000000c 6256 2974 2687 0x00000000
[ 248.208929] 9c7ebe90 00000086 6b67ef8b 0000000c 9f25a610 81a69869 00000001 820b6990
[ 248.216123] 820b6990 820b6990 9c6e4c20 9c6e4eb4 82c78990 00000000 6b993559 0000000c
[ 248.220616] 9c7ebe90 8105f22a 9c6e4eb4 9c6e4c20 00000001 9c7ebe98 9c7ebeb4 81a65cb3
[ 248.229990] Call Trace:
[ 248.234049] [<81a69869>] ? _spin_unlock_irqrestore+0x22/0x37
[ 248.239769] [<8105f22a>] ? prepare_to_wait+0x48/0x4e
[ 248.244796] [<81a65cb3>] rcu_barrier_cpu_hotplug+0xaa/0xc9
[ 248.250343] [<8105f029>] ? autoremove_wake_function+0x0/0x38
[ 248.256063] [<81062cf2>] notifier_call_chain+0x49/0x71
[ 248.261263] [<81062da0>] raw_notifier_call_chain+0x11/0x13
[ 248.266809] [<81a0b475>] _cpu_down+0x272/0x288
[ 248.271316] [<81a0b4d5>] cpu_down+0x4a/0xa2
[ 248.275563] [<81a0c48a>] store_online+0x2a/0x5e
[ 248.280156] [<81a0c460>] ? store_online+0x0/0x5e
[ 248.284836] [<814ddc35>] sysdev_store+0x20/0x28
[ 248.289429] [<8112e403>] sysfs_write_file+0xb8/0xe3
[ 248.294369] [<8112e34b>] ? sysfs_write_file+0x0/0xe3
[ 248.299396] [<810e4c8f>] vfs_write+0x91/0x120
[ 248.303817] [<810e4dc1>] sys_write+0x40/0x65
[ 248.308150] [<81002d73>] sysenter_do_call+0x12/0x28

This change moves an RCU grace period delay off of the
critical path for CPU-hotunplug operations.

Since RCU callback migration is only performed on
CPU-hotunplug operations, and since the rcu_barrier() race is
provoked only by consecutive CPU-hotunplug operations, it is
not necessary to delay the end of a given CPU-hotunplug
operation.

We can instead choose to delay the beginning of the next
CPU-hotunplug operation.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josht@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.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: <20090819060614.GA14383@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
kernel/rcupdate.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 8df1156..bd5d5c8 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -238,7 +238,8 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
call_rcu_bh(rcu_migrate_head, rcu_migrate_callback);
call_rcu_sched(rcu_migrate_head + 1, rcu_migrate_callback);
call_rcu(rcu_migrate_head + 2, rcu_migrate_callback);
- } else if (action == CPU_POST_DEAD) {
+ } else if (action == CPU_DOWN_PREPARE) {
+ /* Don't need to wait until next removal operation. */
/* rcu_migrate_head is protected by cpu_add_remove_lock */
wait_migrated_callbacks();
}