rcu: Permit call_rcu() from CPU_DYING notifiers

As of:

  29494be71afe ("rcu,cleanup: simplify the code when cpu is dying")

RCU adopts callbacks from the dying CPU in its CPU_DYING notifier,
which means that any callbacks posted by later CPU_DYING notifiers
are ignored until the CPU comes back online.

A WARN_ON_ONCE() was added to __call_rcu() by:

  e56014000816 ("rcu: Simplify offline processing")

to check for this condition.  Although this condition did not trigger
(at least as far as I know) during -next testing, it did recently
trigger in mainline:

  https://lkml.org/lkml/2012/4/2/34

What is needed longer term is for RCU's CPU_DEAD notifier to adopt any
callbacks that were posted by CPU_DYING notifiers, however, the Linux
kernel has been running with this sort of thing happening for quite
some time.  So the only thing that qualifies as a regression is the
WARN_ON_ONCE(), which this commit removes.

Making RCU's CPU_DEAD notifier adopt callbacks posted by CPU_DYING
notifiers is a topic for the 3.5 release of the Linux kernel.

Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 0 insertions, 1 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d3922c..d0c5baf1ab18 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1820,7 +1820,6 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
          * a quiescent state betweentimes.
          */
         local_irq_save(flags);
-        WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
         rdp = this_cpu_ptr(rsp->rda);
 
         /* Add the callback to our list. */