tree 73428522f81fab6c4f5797d68a3930eeede67b52
parent 8f20a5e83d2c5d0e126a2fc9bca67f7430dac907
author Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1407241415 -0700
committer Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1410132452 -0700

rcu: Remove redundant preempt_disable() from rcu_note_voluntary_context_switch()

In theory, synchronize_sched() requires a read-side critical section
to order against.  In practice, preemption can be thought of as
being disabled across every machine instruction, at least for those
machine instructions that are not in the idle loop and not on offline
CPUs.  So this commit removes the redundant preempt_disable() from
rcu_note_voluntary_context_switch().

Please note that the single instruction in question is the store of
zero to ->rcu_tasks_holdout.  The "if" is simply a performance optimization
that avoids unnecessary stores.  To see this, keep in mind that both
the "if" condition and the store are in a quiescent state.  Therefore,
even if the task is preempted for a full grace period (presumably due
to its having done a context switch beforehand), the store will be
recording a legitimate quiescent state.

Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Conflicts:
	include/linux/rcupdate.h